Preheat your oven to 350°F (175°C). Grease two 9-inch round cake pans and line with parchment paper.

Chop dark chocolate and melt in a microwave-safe bowl; stir until smooth and set aside to cool.

Cream butter, granulated sugar, and brown sugar in a mixing bowl for about 3 minutes until light and fluffy.

Add eggs one at a time, mixing well between each addition, then add vanilla extract.

Pour the cooled melted chocolate into the mixture and stir until fully incorporated.

Whisk together flour, cocoa powder, baking soda, baking powder, espresso powder (if using), and salt in a separate bowl.

Gradually add the dry ingredients to the wet mixture, alternating with buttermilk until just combined.

Fold in chocolate chips gently using a spatula.

Divide batter between the cake pans and smooth the tops. Bake for 30-35 minutes.

Cool the cakes in the pans for 10 minutes, then invert onto wire racks to cool completely.

For ganache, heat heavy cream until simmering, pour over chopped chocolate, stir until smooth, and add butter and vanilla.

Spread ganache between cooled cake layers and pour remaining ganache on top, allowing it to drizzle down the sides.