In a medium pot, add about 1-inch of water and bring to a simmer.
Add the Egg (7) and Granulated Sugar (2 cup) to the bowl of a stand mixer and whisk together. Place mixing bowl over pot of simmering water. Whisk constantly until sugar is dissolved into the egg whites, mixture should reach 160 degrees F (70 degrees C) (takes about 3-5 minutes). Place bowl in the freezer for 10-15 minutes to cool a bit.
Place bowl into your stand mixer fitted with whisk attachment. Beat on medium-high speed until stiff glossy peaks form (about 10-15 minutes), the bottom of the bowl feels at room temperature and not hot or warm. You can wait for it to cool down before adding Unsalted Butter (1 1/2 cup).
Once bowl is at room temperature, switch to paddle attachment, reduce to medium speed and add butter 1 tablespoon at a time. Once all butter is in, increased speed to high and beat the buttercream until has reached a thick whipped consistency (3-5 minutes). If the buttercream is liquidy, place in the fridge for 30-60 minutes then whip.
Add Pure Vanilla Extract (1 tablespoon) and mix until is well blended. Set aside Swiss Meringue Buttercream until ready to decorate.
Preheat oven to 350 degrees F (180 degrees C). Line two cake pans with parchment paper or butter and flour the pans. In a medium bowl sift All-Purpose Flour (2 cup), Baking Powder (2 teaspoon), Baking Soda (1/2 teaspoon) and Salt (1/2 teaspoon). Set aside
In a large bowl, cream together Butter (1/2 cup), Caster Sugar (1 cup) and Oil (1/4 cup) until fluffy. Add Pure Vanilla Extract (1 tablespoon) and Large Egg (3), add the eggs 1 at a time and mix. Mix in Sour Cream (1/2 cup) and Milk (1/2 cup). Add flour mixture and whisk until there’s no lumps.
Divide the cake batter in the cake pans. Spread eventually with a spatula. Bake for 30-35 minutes or until a toothpick inserted in the middle comes out clean. (If you use 3 cake pans the cakes will be done in 20-25 minutes)
Once cakes are baked, allow them to be on the pan for 5 minutes then remove from pan. Allow cakes to cool completely before adding any kind of frosting you prefer.
Frost and decorate the cake based with the Swiss Meringue Buttercream. Serve and enjoy!