- Preheat oven to 350°F. Grease two 8-inch round pans with butter, line bottoms with parchment, butter again, and flour lightly.
- In a stand mixer bowl, sift flour, sugar, cocoa, baking soda, baking powder, and salt. Mix on low with paddle attachment to combine.
- In a separate bowl, whisk buttermilk, oil, eggs, and vanilla.
- With mixer on low, slowly pour wet mixture into dry. Add hot coffee and mix just until combined, scraping down as needed.
- Pour into prepared pans and bake 35–40 minutes, until a toothpick comes out clean.
- Cool in pans for 30 minutes, then invert onto racks to cool completely.
Frosting:
- Melt chocolate over a double boiler; stir until smooth and let cool slightly.
- Beat butter in mixer on medium-high for 3 minutes until pale and fluffy. Add vanilla and egg yolk, and beat another 3 minutes.
- Turn to low speed and slowly add powdered sugar. Once incorporated, beat on medium until silky and smooth.
- Dissolve coffee granules in 2 teaspoons hot water. With mixer on low, add coffee and melted chocolate. Mix until fully blended.
- Frost cooled cakes generously. Slice. Serve. Swoon.
