Preheat your oven to 350°F (177°C). Grease four 8-inch or 9-inch cake pans, then line each with parchment paper rounds and grease the parchment as well. This helps the cakes release smoothly from the pans. (If you need help, there’s a helpful video and post on making parchment paper rounds for cakes.)

To make the cake batter, whisk together the flour, sugar, cocoa powder, espresso powder (if using), baking soda, baking powder, and salt in a large bowl and set aside. In another large bowl, use a handheld or stand mixer with a whisk attachment—or a regular whisk—to beat the sour cream, oil, and eggs on medium-high speed until combined. Add the buttermilk and vanilla extract and mix again until smooth. Slowly add the dry ingredients to the wet ingredients, then pour in the hot water or coffee and mix on low speed until fully incorporated. If using, fold in the mini chocolate chips that have been tossed in flour. Divide the batter evenly among the prepared pans.

Bake the cakes for 19 to 23 minutes, keeping an eye on them since oven times can vary. They’re done when a toothpick inserted in the center comes out clean. Let the cakes cool in their pans on a wire rack for about 1 hour. To remove, run a knife around the edges, then carefully take the cakes out, peel off the parchment paper, and let them finish cooling on the rack. It’s normal if the centers sink slightly as they cool.

For the chocolate mousse, whisk the hot water and cocoa powder together in a small bowl until smooth. Melt the chopped chocolate using a double boiler or microwave. If microwaving, heat the chocolate in 20-second intervals, stirring well between each until fully melted and smooth. Stir the hot water and cocoa mixture into the melted chocolate until thick and silky. In a large bowl, whip the cold heavy cream, confectioners’ sugar, and vanilla extract on medium-high speed until medium peaks form—this should take about 3 minutes. Gently fold the chocolate mixture into the whipped cream with a spatula, being careful not to overmix so you don’t deflate the mousse. Cover and refrigerate for at least 2 hours or up to 2 days. This will yield about 4.5 cups (900g) of mousse.

Before assembling, level the cakes if needed by slicing a thin layer off the tops with a serrated knife. Place the first cake layer on a cake stand, turntable, or serving plate. Spread about 1.5 cups (200g) of chocolate mousse evenly over the top. Add the second layer upside down and spread another 1.5 cups of mousse on top. Repeat with the third layer (right side up), again covering with mousse. Place the final cake layer on top, then spread the remaining mousse over the top and sides of the cake as a crumb coat. Smooth the sides using a bench scraper. Chill the cake in the refrigerator for at least 1 hour and up to 4 hours while you prepare the ganache.

To make the ganache, place the chopped chocolate in a medium heat-proof bowl. Heat the cream in a small saucepan over medium heat until it just starts to simmer—avoid boiling. Pour the hot cream over the chocolate and let it sit for 2 to 3 minutes to soften the chocolate. Stir slowly with a metal spoon or small spatula until smooth and fully melted. The finer the chocolate is chopped, the faster it will melt. If the ganache isn’t melting properly, do not microwave it; check troubleshooting tips if needed. Chill the ganache in the refrigerator for at least 30 minutes or up to 1 hour to thicken.

Once thickened, pour or spoon the ganache over the chilled cake and smooth the top and sides with an icing spatula. Garnish with fresh berries and/or chocolate shavings if desired. Serve the cake immediately or chill it uncovered for 4 to 6 hours before serving. The cake can be enjoyed at room temperature or chilled. Cover any leftovers tightly and store in the refrigerator for up to 5 days.