Let’s embark on creating this spectacular Funfetti Cake! It’s a project, so break it down into stages: Cake, IMBC, American BC, Ganache, Assembly.

Make the Cake Layers: Preheat oven to 340°F (170°C – note unusual temp). Grease and flour three 6-inch round cake pans, line bottoms with parchment. Optional: wrap pans with cake strips. In a large bowl, sift together the flour, granulated sugar, baking soda, baking powder, and salt. Repeat sifting if desired for extra lightness.

In a medium bowl, whisk together the room temperature butter (ensure very soft if using “add wet to dry” method, otherwise cream it first if preferred), egg whites, vanilla, sour cream, and milk. Correction: Recipe implies adding combined wet to combined dry. So, whisk wet ingredients: egg whites, vanilla, sour cream, milk. In stand mixer bowl (or large bowl), combine sifted dry ingredients. With mixer on low (or by hand), gradually add the combined wet ingredients to the dry ingredients, mixing just until combined. Be careful not to overmix. Gently fold in the 1/3 to 1/2 cup sprinkles using a spatula, mixing as little as possible.

Divide batter evenly among the three prepared pans. Bake for 30-35 minutes, or until centers spring back lightly when touched and a toothpick inserted comes out clean. Let cakes cool in pans on wire racks for 10-15 minutes. Carefully invert cakes onto racks, peel off parchment, and let cool completely. Once cool, level tops if needed, wrap well, and chill if desired for easier assembly.

Make the Italian Meringue Buttercream (IMBC): Place the 5 room temperature egg whites, 2 dashes cream of tartar, and pinch of salt in the meticulously clean bowl of your stand mixer fitted with the whisk attachment. Add 1/3 cup of the granulated sugar. Beat on medium-high speed until soft peaks form.

Meanwhile, in a medium saucepan, combine the remaining 1 cup granulated sugar and 1/3 cup water. Stir over low heat until sugar dissolves completely and liquid is clear. Increase heat to medium-high, attach candy thermometer, and boil without stirring until syrup reaches 235-240°F (113-115°C – soft-ball stage). Wash down pan sides with wet pastry brush as needed.

As soon as syrup reaches temperature, remove from heat. With the mixer running on medium-low speed, very carefully and slowly drizzle the hot syrup in a thin stream down the side of the bowl into the whipping egg whites, avoiding the whisk itself. Once all syrup is added, increase speed to medium-high and beat until the meringue is thick, glossy, and the outside of the bowl feels completely cool to the touch (this can take 10-15 minutes).

Switch to the paddle attachment. With the mixer on low speed, add the room temperature butter, one piece at a time, allowing each to incorporate slightly before adding the next. The mixture might look curdled or soupy – don’t panic! Keep beating on medium-high speed until it suddenly comes together into a smooth, silky buttercream (can take 5-10 more minutes). Beat in the 2 tsp vanilla extract. Set aside (or refrigerate briefly if too soft, then re-whip).

Make the Funfetti American Buttercream: In a large bowl (stand mixer with paddle preferred), beat the 1 lb (4 sticks) room temperature butter until light and fluffy. Gradually add the 2 lb sifted powdered sugar, mixing on low speed until incorporated, alternating with tiny splashes of the 1 tsp milk if needed. Add the 1/4 tsp salt and 3 tsp vanilla; beat until smooth.

Divide the frosting into two batches. Add 1 drop (or more) pink food coloring to one batch and mix until desired color is reached. Gently fold 1/6 cup sprinkles into the pink batch and 1/6 cup sprinkles into the white batch. Prepare a large piping bag for two-tone effect (e.g., lay plastic wrap, spread pink on one side, white on other, roll up, place in bag) or use separate bags.

Make the Ganache: Finely chop candy melts if large. Place chopped candy melts and white chocolate chips in a microwave-safe bowl with the 3 tbsp heavy cream. Microwave on medium power (50%) in 20-30 second intervals, stirring well each time, until melted and smooth. Whisk until no lumps remain (reheat very gently if needed). Stir in a drop of pink food coloring if desired. Let ganache cool slightly until it reaches a pourable consistency suitable for dripping (test on side of a cold glass). Transfer to a piping bag or squeeze bottle.

Assemble the Cake: Place one cooled cake layer on your cake stand/board. Pipe a dam of plain IMBC (or some American BC) around the edge if desired (to hold filling). Fill the layer with the two-tone Funfetti American Buttercream, piping alternating rings or however you loaded your bag. Smooth lightly.

Place the second cake layer on top. Repeat piping the Funfetti American Buttercream filling. Place the third cake layer on top (upside down for flat top). Apply a thin crumb coat using alternating pink and white Funfetti American Buttercream (or just plain white/pink, or leftover IMBC). Smooth with offset spatula/bench scraper. Chill cake for 10-30 minutes to set crumb coat.

Apply the final coat of Italian Meringue Buttercream smoothly all over the chilled cake exterior. Use an offset spatula and bench scraper for smooth sides and sharp top edge. Chill cake again for at least 10-30 minutes to firm up frosting.

Gently press confetti sprinkles around the bottom edge of the cake to create a “sprinkle skirt.” Carefully apply the slightly cooled pink ganache around the top edge, letting it drip down the sides naturally. Smooth ganache on top if desired. Let ganache set slightly (chill briefly if needed).

Fit a piping bag with a decorative tip (like #869 French star) and fill with remaining IMBC. Pipe decorative dollops or swirls around the top edge of the cake, over the ganache border. Chill the finished cake for at least 30-60 minutes before serving to set everything. Let stand at room temp 15-20 min before slicing.