For chocolate cake: Preheat oven to 325 degrees. Spray 3 (6-inch) round cake pans with vegetable oil. Line each with a parchment circle on the bottom and spray once more. Set aside.
In a large mixing bowl, combine all dry ingredients. Set aside. In another bowl, combine all liquid ingredients (eggs, coffee, oil and buttermilk). Add liquids to dry ingredients, stirring until just combined. Fill each pan one-quarter way, about 1 to 2 cups batter for each pan, and bake 15 to 20 minutes or until cake springs back when lightly touched or when a toothpick comes out clean. Extra batter should be baked separately to create a crumb finish for the cake.
For blackberry mousse: In a saucepan, add 1 cup blackberries and 1 tbsp. sugar. Cook on medium heat until berries are soft. Remove from heat and strain, getting as much juice as possible. Discard softened berries and seeds and set juice aside to let cool.
In a stand mixer using a whisk attachment, add cream, remaining 1/4 cup sugar and cooled blackberry juice. Whip on medium-high speed until medium-stiff peaks form. Keep chilled until ready to use.
To assemble: Take one layer of chocolate cake and fill with a good amount of blackberry mousse (fillings should be of equal or lesser size of actual cake layer). Top with a few fresh berries and repeat for the two remaining layers. Next, add a thin layer of blackberry mousse around the entire cake and chill for 15 minutes. Once chilled, add a final coat of blackberry mousse. Add cake crumbs to sides and garnish with fresh blackberries on top.
Makes 1 (6-inch, 3-layer) cake