1. Prepare four 8 inch cake pans with parchment paper circles in the bottom and grease the sides. Preheat oven to 350°F (176°C).
2. Combine the flour, baking powder and salt in a medium sized bowl and set aside.
3. Add the butter, sugar, oil and vanilla extract to a large mixer bowl and beat together until light in color and fluffy, about 3-4 minutes. Do not skimp on the creaming time.
4. Add the eggs one at a time, mixing until mostly combined after each. Scrape down the sides of the bowl as needed to be sure all ingredients are well incorporated.
5. Add half of the dry ingredients to the batter and mix until mostly combined.
6. Slowly add the milk and mix until well combined. The batter will look curdled, but that’s ok.
7. Add the remaining dry ingredients and mix until well combined and smooth. Scrape down the sides of the bowl as needed to be sure all ingredients are well incorporated. Do not over mix the batter.
8. Divide the batter evenly between the cakes pans and bake for 17-20 minutes, or until a toothpick comes out with a few crumbs.
9. Remove the cakes from the oven and allow to cool for about 2-3 minutes, then remove to cooling racks to cool completely.

TO MAKE THE TIRAMISU FILLING:

10. When the cakes have cooled, make the filling. Combine the egg yolks and sugar in the top of a double boiler (or in a metal mixer bowl over a pot of simmer water). Whisk to combine. The mixture will seem a little chunky at first, but will thin out as it heats up.
11. Continue cooking and whisking continuously for about 5-8 minutes, until mixture grows in volume, thickens and turns a pale yellow. The sugar should melt, but if it’s still a little grainy, that’s ok. It will smooth out as you go. The temperature of the mixture should reach about 155°F (68°C).
12. Set the egg mixture aside to cool to a little warmer than room temperature.
13. While eggs cool, make the whipped cream. Add the heavy whipping cream to a large mixer bowl and whip on high speed until stiff peaks form.
14. When the egg mixture has cooled, add the mascarpone cheese and gently fold together until combined.
15. Gently fold the whipped cream into mascarpone/egg mixture in two parts. Set mixture aside.

TO MAKE THE FROSTING:

16. Place the butter in a large mixer bowl and beat until smooth.
17. Add about half of the powdered sugar and mix until well combined and smooth.
18. Add the mascarpone cheese and mix until well combined and smooth.
19. Add the salt and vanilla extract and mix until well combined and smooth.
20. Add the remainder of the powdered sugar and mix until well combined and smooth. Add some water or cream, if needed to thin out the frosting a bit.

TO LAYER THE CAKE:

21. To put together the espresso Kahlua mixture, combine the hot water, espresso powder and sugar and stir until well combined. Add the Kahlua and stir together. Set aside.
22. Use a large serrated knife to remove the domes from the top of the cakes so that they’re flat.
23. To build the cake, place the first layer of cake on a cardboard cake circle or a serving platter. Drizzle 4-5 tablespoons of the espresso mixture over the cake layer. (For help with building a layer cake, check out my tutorial)
24. Use the mascarpone frosting to pipe a dam around the outer edge of the cake layer, then fill it with about 1/3 of the tiramisu filling. Spread evenly.
25. Add the second layer of cake, another 4-5 tablespoons of espresso mixture and another filling layer.
26. Repeat with the third layer of cake, espresso mixture and filling.
27. Add the final layer of cake on top and then add a crumb coat of frosting (a very thin layer). Put the cake in the fridge for 30 minutes to an hour if it seems like it needs to firm up a bit.
28. Frost the outside of the cake. Refer to my tutorial for frosting a smooth cake, if needed.
29. Dust the top of the cake with some cocoa powder, then use the remaining frosting to pipe a border around the top and bottom edges. I used Ateco tip 844. Finish off the cake with some chocolate covered espresso beans.
30. Store the cake in an airtight container in the fridge prior to serving. Cake is best if eaten within 2-3 days.