• Preheat your oven to 325°F (163°C). Grease and flour three 8-inch round cake pans (or opt for two 9-inch pans). Line each pan base with a parchment round for easy removal.
• In a small bowl, dissolve the espresso powder in hot water and set aside to cool slightly.
• In a large mixing bowl, combine the cake mix, flour, sugar, and pudding mix, whisking to blend evenly. Add the eggs, sour cream, vegetable oil, and the prepared espresso mixture. Using an electric mixer (fitted with a paddle if using a stand mixer), mix on low speed to moisten ingredients, then increase to medium speed, mixing for 2 minutes, scraping down sides as needed.
• Evenly distribute the batter among the prepared cake pans and bake for 22 minutes or until a toothpick inserted comes out with a few moist crumbs, or the center of each layer springs back lightly to touch. Once done, let the layers cool for 5 minutes in their pans, then turn out onto wire racks to cool completely.

Mocha Mousse Filling

• In a small bowl, dissolve the espresso powder in two tablespoons of heavy cream. In a larger bowl, combine the pudding mix, espresso mixture, and remaining heavy cream. Using a hand mixer or whisk, mix on low to blend, then whip on medium speed until the mousse is thick and fluffy. If needed, thin with a bit more cream to reach desired consistency.

Mocha Buttercream Frosting

• In a cup, dissolve the instant espresso powder in ¼ cup of hot water; set aside to cool. In a mixing bowl, beat the softened butter on medium speed until creamy. Add vanilla, cocoa powder, and salt, mixing until smooth. Add half of the powdered sugar and half of the coffee mixture, blending on medium until incorporated. Add the remaining sugar and coffee, beating for an additional 3-6 minutes, scraping down the bowl as needed. Adjust the consistency as desired by adding more powdered sugar or a teaspoon of water or milk.

Assembling the Cake

• Once the cake layers are fully cooled, begin assembly. Place the first cake layer on a cake plate or stand, spread with a layer of mocha mousse filling, and repeat with the second layer. Top with the final layer, filling in any gaps between layers with a light layer of buttercream. Apply a thin crumb coat of frosting, then chill the cake for 15 minutes in the freezer or longer in the refrigerator. Finish with a smooth layer of frosting and decorate as desired!