Start by preparing four 8-inch cake pans: line the bottoms with parchment paper circles and lightly grease the sides. Preheat your oven to 350°F (176°C). In a medium bowl, whisk together the flour, baking powder, and salt, then set aside. In a large mixing bowl, combine the butter, sugar, oil, and vanilla extract. Beat the mixture on medium-high speed for about 3–4 minutes until it becomes pale and fluffy—don’t rush this step, as the creaming process is essential for a light cake texture. Add the eggs one at a time, mixing after each until just combined, and scrape down the sides of the bowl as needed to ensure everything is incorporated. Add half of the dry ingredients to the batter and mix until just combined. Slowly pour in the milk while mixing on low speed until the batter is fully combined. It may look slightly curdled at this point, which is completely normal. Add the remaining dry ingredients and mix again until the batter is smooth and well blended, being careful not to overmix. Evenly divide the batter among the prepared cake pans and bake for 17–20 minutes, or until a toothpick inserted in the center comes out with just a few moist crumbs. Remove the cakes from the oven and let them cool in the pans for 2–3 minutes before transferring to wire racks to cool completely.

Once the cake layers have cooled, begin preparing the tiramisu filling. In the top of a double boiler (or a heatproof metal bowl set over a pot of gently simmering water), whisk together the egg yolks and sugar. The mixture might look grainy at first but will smooth out as it heats. Cook the mixture while whisking constantly for 5–8 minutes, until it thickens, becomes pale yellow, and increases in volume. It should reach an internal temperature of about 155°F (68°C). If it’s still slightly grainy, that’s fine—it will continue to smooth out as you proceed. Remove from heat and allow it to cool to slightly warmer than room temperature. While the egg mixture is cooling, whip the cold heavy cream in a separate large bowl until stiff peaks form. Once the egg mixture has cooled, gently fold in the mascarpone cheese until evenly combined. Then fold in the whipped cream in two additions, gently mixing until smooth and fluffy. Set the finished tiramisu filling aside.

In a large mixing bowl, beat the softened butter until smooth and creamy. Add about half of the powdered sugar and mix on low speed until combined, then increase to medium-high and beat until smooth. Add in the mascarpone cheese and continue mixing until fully incorporated. Stir in the salt and vanilla extract and mix until smooth. Add the remaining powdered sugar and beat again until the frosting is well blended and fluffy. If needed, add 1 teaspoon to 1 tablespoon of water or cream to adjust the consistency to your liking.

To prepare the espresso Kahlua mixture, stir together the hot water, instant espresso powder, and sugar until dissolved. Add the Kahlua and mix well. Set aside. Using a large serrated knife, level the tops of the cake layers by trimming off any domes so the layers are flat. Place the first cake layer on a cake board or serving plate. Drizzle 4–5 tablespoons of the espresso mixture over the top. Pipe a frosting dam around the edge of the cake to hold in the filling, then spread about one-third of the tiramisu filling inside the dam and smooth it out evenly. Add the second cake layer and repeat the process: espresso mixture, frosting dam, and filling. Do the same with the third layer. Place the final cake layer on top and apply a thin crumb coat of frosting over the entire cake. Chill the cake in the fridge for 30 minutes to an hour, especially if the layers or filling feel soft. Once chilled, frost the entire cake with the remaining mascarpone buttercream, smoothing the sides and top. Lightly dust the top with cocoa powder, then pipe a decorative border around the top and bottom edges using any piping tip you prefer—Ateco tip 844 was used for this version. Finish by adding chocolate-covered espresso beans as a garnish. Store the completed cake in the fridge in an airtight container. For best flavor and texture, serve within 2–3 days.