Preheat oven to 350°F (180°C). Line 2 (8-inch) round cake pans with parchment paper; spray with cooking spray.
In a large bowl, whisk together milk, oil, and eggs. Fold in flour, sugars, cocoa, baking soda, baking powder, and salt. Stir in sour cream and vanilla until combined. Divide batter between prepared pans.
Bake until a wooden pick inserted in center comes out clean, 30 to 35 minutes. Let cool completely in pans. Turn out cakes onto a wire rack.
Using a serrated knife, cut cakes in half horizontally. Place one cake layer on a cake plate. Pour one-fourth of coffee liqueur over cake. Spread one-fourth of Peanut Butter Mousse onto cake layer. Top with a second cake layer, and repeat procedure. Repeat with remaining cake layers, remaining coffee liqueur, and remaining Peanut Butter Mousse. Garnish with peanuts and chocolate morsels, if desired.
Recipe by Bake from Scratch at https://bakefromscratch.com/kahlua-chocolate-cake-peanut-butter-mousse/