Preheat oven to 350°F.

Grease three 8-inch cake pans and line the bottoms with parchment paper.

Whisk together the flour, baking powder, baking soda, salt, and Dutch-process cocoa powder.

In a separate large bowl, combine the granulated sugar, dark brown sugar, eggs, avocado oil, sour cream, buttermilk, and vanilla.

Stir the espresso powder into the boiling water.

Gradually combine the wet and dry ingredients, then add the hot espresso water and mix until you have a smooth chocolate batter.

Divide the batter evenly between the three prepared pans.

Bake all three cakes on the middle rack for about 25–28 minutes.

Start checking at 25 minutes.

Allow the cakes to cool before removing them from the pans, then cool completely before assembling.

Combine the strong coffee, dark brown sugar, Dutch-process cocoa powder, vanilla, and a pinch of salt.

Stir until the sugar and cocoa are completely dissolved, then let it cool.

Brush or spoon the soak evenly over the cooled cake layers and allow it to absorb.

Finely chop the semisweet and dark chocolate.

Heat the heavy cream until hot and steaming, then pour it over the chopped chocolate.

Let it sit briefly, then stir until completely smooth.

Beat in the butter, Dutch-process cocoa, regular 100% cocoa, sifted powdered sugar, vanilla, espresso powder, and salt until thick, smooth, and glossy.

If the frosting is too soft, let it sit or chill briefly until it reaches a thick, spreadable consistency.

Place the first cake layer down and add a generous layer of fudge frosting.

Repeat with the second layer.

Add the final cake layer and cover the entire cake with the remaining chocolate fudge frosting.