Preheat your oven to 350°F (175°C). Prepare two 9-inch round cake pans by lining the bottoms with parchment paper and lightly greasing the sides.

In a microwave-safe bowl, melt the chopped dark chocolate until just smooth, stirring every 30 seconds. Allow to cool slightly.

In a large mixing bowl, combine room-temperature unsalted butter, granulated sugar, and light brown sugar. Beat on medium speed for about 3 minutes until fluffy.

Add the large eggs one at a time, ensuring each is fully incorporated before adding the next. Mix in the pure vanilla extract.

Gently fold the melted chocolate into the butter mixture until fully combined.

In a separate bowl, whisk together the all-purpose flour, Dutch-processed cocoa powder, baking soda, baking powder, espresso powder (if using), and salt.

Alternately add the dry ingredients and buttermilk to the chocolate mixture, starting and ending with the dry ingredients.

Fold in the semi-sweet chocolate chips.

Divide the batter between the prepared cake pans and bake for 30-35 minutes, or until a toothpick comes out with moist crumbs.

Cool the cakes in the pans for about 10 minutes, then transfer to a wire rack to cool completely.

For the ganache, heat the heavy cream until just simmering, remove from heat, and pour over chopped chocolate. Let sit for 2 minutes.

Stir in additional butter and a splash of vanilla extract until smooth.

Once cooled, assemble the cake by placing one layer on a serving platter, spreading ganache on top, adding the second layer, and pouring remaining ganache over the cake.