Preheat and Prep: Preheat oven to 350°F (175°C) and line 2 muffin tins with liners.

Combine Dry Ingredients: In a large bowl, whisk together the flour, instant espresso, salt, baking powder, and baking soda.

Cream Butter and Sugar: In another large bowl using a hand mixer, beat the softened butter and sugars together until light and fluffy.

Add Sour Cream: Add the room-temperature sour cream and beat until incorporated.

Add Eggs and Vanilla: Scrape down the sides of the bowl, then add the eggs one at a time, beating well after each addition, then add the vanilla extract.

Combine Dry and Wet Ingredients: Add the dry ingredients to the wet ingredients and beat until just combined. Be careful not to overmix.

Add Coffee: Add the freshly brewed coffee and beat until just combined.

Fill Cupcake Liners: Using a large cookie scoop (or a spoon), fill the cupcake liners ¾ full with batter.

Bake: Bake until a toothpick inserted in the middle comes out clean, about 20 minutes.

Cool: Let the cupcakes cool completely in the muffin tins before frosting.

Make Frosting: While the cupcakes are cooling, make the frosting. In a large bowl using a hand mixer (or in the bowl of a stand mixer with the whisk attachment), beat the softened butter until light and fluffy and pale in color, about 3 minutes.

Add Condensed Milk: With the mixer running on medium speed, slowly add the chilled condensed milk.

Add Vanilla and Salt: Add the vanilla extract and salt and beat until the frosting is stiff.

Frost Cupcakes: Transfer the frosting to a piping bag fitted with a tip (or use a spoon or offset spatula) and pipe or spread the frosting onto the cooled cupcakes.