Prep the Pan and Oven: Heat the oven to 350°F (175°C). Grease a 9-inch round cake pan, line the bottom with parchment, and lightly flour the sides. This helps the cake release cleanly.

Combine Dry Ingredients: In a large bowl, whisk flour, cocoa powder, baking powder, baking soda, and salt. Breaking up cocoa clumps now prevents streaks later.

Whisk Wet Ingredients: In a second bowl, whisk granulated sugar, brown sugar, eggs, oil, sour cream, and vanilla until smooth. Stir in espresso powder if using.

Bring It Together: Add the dry mixture to the wet and stir gently. Pour in the hot coffee or water and whisk just until smooth. The batter will be fairly thin—that’s good for moisture.

Bake: Pour into the pan and bake 28–34 minutes, or until a toothpick comes out with a few moist crumbs. Do not overbake. Cool in the pan 10 minutes, then invert onto a rack and cool completely.

Make the Ganache: Place chopped chocolate in a heatproof bowl. In a small saucepan, heat cream until it just begins to steam with small bubbles at the edges. Pour over the chocolate, cover the bowl, and let sit 3 minutes.

Finish the Ganache: Add butter, salt, and vanilla. Stir slowly from the center out until glossy and smooth. If pieces remain, warm the bowl over a simmering pot of water for 10–15 seconds and stir again. You want a thick, pourable consistency.

First Coat (Crumb Coat): Set the cooled cake on a stand. Spread a thin layer of ganache over the top and sides to trap crumbs. Chill 15–20 minutes to set.

Final Coat: Spread the remaining ganache smoothly over the cake. For a truffle look, keep the sides straight and the top glossy. Chill 30–45 minutes until the ganache is softly set.

Finish and Serve: Dust with cocoa, add chocolate shavings, or garnish with berries. Slice with a warm, dry knife for clean edges. Serve slightly cool or at room temperature.