Preheat your oven to 350°F (175°C). Line and grease two 9-inch round cake pans with parchment paper and unsalted butter.

Melt the dark chocolate gently in a heatproof bowl using a microwave, stirring until smooth. Let it cool slightly.

In a mixing bowl, cream room temperature unsalted butter with granulated sugar and light brown sugar until light and fluffy.

Add the large eggs one at a time, incorporating fully before adding the next. Then mix in the pure vanilla extract and melted chocolate.

Whisk together all-purpose 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, mixing gently until just combined.

Fold in the semi-sweet chocolate chips using a spatula until evenly distributed.

Divide the batter between the prepared cake pans and bake for 30-35 minutes. A toothpick inserted should come out with moist crumbs.

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

Heat heavy cream until simmering, pour over chopped dark chocolate, whisk in the unsalted butter and vanilla extract until smooth.

Place one cooled cake layer on a serving plate, spread ganache in between layers, then cover the top and sides with remaining ganache.