Step 1: Preheat and Prep
Preheat the oven to 350°F (180°C). Line an 8×8-inch pan with parchment paper, letting it hang over the sides for easy removal.

Step 2: Mix Dry Ingredients
In a mixing bowl, rub the lemon zest into the sugar using your fingers until the sugar is fragrant.

Add the cake flour, baking powder, and salt. Whisk to combine evenly.

Step 3: Cut in the Butter
Add the cubed butter to the dry mixture. Mix on low speed until the texture resembles damp sand.

This step ensures the crumb of the cake will be soft and tender.

Step 4: Add Wet Ingredients
In another bowl, whisk together the milk, sour cream, canola oil, egg, and vanilla.

Slowly pour the wet mixture into the dry ingredients. Mix until just combined. Avoid overmixing to keep the cake tender.

Step 5: Bake the Cake
Pour the batter into the prepared pan and smooth the top.

Bake for 23–26 minutes, or until a toothpick inserted into the center comes out clean.

Cool the cake completely on a wire rack before frosting.

Step 6: Make the Frosting
In a mixing bowl, beat the cold mascarpone with powdered sugar until smooth.

Add the heavy cream, vanilla, and lemon zest. Whip on low to combine, then increase to medium speed and whip until stiff peaks form.

Don’t overmix—the frosting should be thick but smooth and creamy.

Step 7: Prepare the Berries
Gently toss the blackberries, chopped strawberries, and raspberries with the jam in a bowl.

The jam adds sweetness and shine to the berries.

Step 8: Assemble the Cake
Spread the mascarpone frosting over the cooled cake, right to the edges.

Top with the prepared berries. If not serving immediately, wait to add the berries until just before serving.

Slice and serve cold or at room temperature.