Preheat the oven to 350°F (177°C). Grease four 8-inch or 9-inch cake pans, line with parchment paper rounds, then grease the parchment paper.

In a large bowl, whisk the flour, sugar, cocoa powder, espresso powder, baking soda, baking powder, and salt together. Set aside.

In another large bowl using a handheld or stand mixer fitted with a whisk attachment, mix the sour cream, oil, and eggs together on medium-high speed until combined. Add the buttermilk and vanilla and mix until combined. Pour the dry ingredients into the wet ingredients, add the hot water/coffee, and whisk or beat on low speed until combined. Fold in the floured mini chocolate chips, if using. Divide the batter evenly among the prepared pans.

Bake for 19–23 minutes. The cakes are done when a toothpick inserted in the center comes out clean. Cool the cakes in the pans set on a cooling rack for 1 hour. Run a knife around the edges to help loosen the sides, remove the cakes from the pans, peel off the parchment, and place on the rack to finish cooling.

In a small bowl, whisk the hot water and cocoa powder together until smooth. Melt the chopped chocolate in a double boiler or use the microwave. Pour hot water/cocoa mixture into the melted chocolate and stir until thick and smooth. Set aside.

In a large bowl using a handheld or stand mixer fitted with a whisk attachment, whip the heavy cream, confectioners’ sugar, and vanilla extract together on medium-high speed until medium peaks form, about 3 minutes. Pour in the chocolate mixture and gently fold together with a spatula. Cover the mousse and refrigerate for at least 2 hours or up to 2 days.

Level the cakes if needed. Place 1 cake layer on your cake stand. Cover the top with about 1.5 cups of chocolate mousse. Top with the second layer, upside down, and cover with the same amount of mousse. Top with the third layer, right side up, and cover with the same amount of mousse. Top with final cake layer. Spread the remaining mousse on top and around the sides of the cake as a crumb coat. Refrigerate for at least 1 hour.

Make the chocolate ganache: Place the chocolate in a medium heat-proof bowl. In a small saucepan, heat the cream over medium heat just until it begins to gently simmer. Pour the warm cream over the chocolate and let it sit for 2–3 minutes. Slowly stir until the chocolate has melted and the mixture is smooth. Refrigerate the ganache for at least 30 minutes to thicken.

Pour/spoon the thickened ganache on the chilled cake. Smooth the top and sides with an icing spatula. Top with optional garnish such as fresh berries and/or chocolate shavings. Serve cake immediately or chill for up to 4–6 hours before serving.