Preheat your oven to 350 degrees F (175 degrees C). While it's warming up, grab two 8-inch round cake pans. Butter each pan generously. Then, line the bottoms with parchment paper, butter the parchment, and dust with flour to ensure a flawless release after baking.

In the bowl of your electric mixer equipped with a paddle attachment, add the flour, sugar, cocoa powder, baking soda, baking powder, and kosher salt. Mix these together on low speed until well-combined. This step is crucial as it helps to aerate the flour and evenly distribute the cocoa throughout the mixture.

In a separate bowl, whisk together the buttermilk, vegetable oil, eggs, and vanilla extract. Make sure everything is at room temperature for the best emulsification. This helps the ingredients blend beautifully without any lumps.

With your mixer on low speed, gradually pour the wet mixture into the bowl with the dry ingredients. Allow the mixer to do its magic, blending until just combined. Be sure to scrape the sides and the bottom of the bowl with a rubber spatula.

This is where the magic happens! While the mixer is still on low, slowly pour in the freshly brewed hot coffee. This not only enhances the chocolate flavor but also helps create that delectable moist crumb. Just mix until everything is blended; over-mixing can result in a dense cake.

Divide the batter evenly between the prepared pans. Use the spatula to smooth the tops if needed. Bake for about 35 to 40 minutes, or until a cake tester or toothpick inserted into the center comes out clean. Remove from the oven and let the cakes cool in the pans for about 30 minutes. Then, turn them out onto a cooling rack to cool completely.

While the cakes are cooling, let’s whip up that scrumptious chocolate frosting. Start by chopping your semisweet chocolate into small pieces, placing it in a heat-proof bowl set over a pot of simmering water. Stir gently until just melted, then allow it to cool.

In a clean bowl, beat the room-temperature butter on medium-high speed until it looks pale and fluffy, about 3 minutes. Add the egg yolk and vanilla, and continue beating for another 3 minutes. The frosting should be airy and light.

Now, on a low speed, gradually add the sifted confectioners' sugar until incorporated. Dissolve the instant coffee in 2 teaspoons of hot tap water, and then mix this and the melted chocolate into the frosting. Be careful not to whip—just blend until smooth and creamy.

Place one of the cooled cake layers onto a serving plate, flat side up. Using an offset spatula, spread a generous layer of frosting on top. Place the second layer, rounded side up, on top of the frosting. Now, gently spread frosting over the top and down the sides of the cake. It doesn’t have to be perfect; that’s part of its charm!