Ingredients:
- 1 box (15.25 oz / 432 g) Yellow or White Cake Mix
- 3 whole Large Eggs (room temperature)
- 2 Large Egg Yolks (room temperature)
- ½ cup (1 stick / 113 g) Unsalted Butter (melted and slightly cooled)
- 1 cup (240 ml) Whole Milk (full-fat, room temperature)
- ½ cup (120 g) Sour Cream or Greek Yogurt (full-fat, room temperature)
- 2 teaspoons Vanilla Extract
- 1 cup (2 sticks / 226 g) Unsalted Butter (softened, for Simple Vanilla Buttercream)
- 4 cups (480 g) Icing Sugar (Powdered Sugar, sifted, for Buttercream)
- 1 teaspoon Vanilla Extract (for Buttercream)
- 2–4 tablespoons Whole Milk or Heavy Cream (for Buttercream)
Instructions:
- Preheat oven to 350°F (175°C). Grease and line the bottoms of two 8-inch cake tins with rounds of parchment paper. Dust the sides lightly with flour.
- Melt the butter and set it aside to cool slightly. In a large bowl or the bowl of a stand mixer, whisk together the 3 whole eggs and 2 extra egg yolks until just combined. Gently mix in the cooled melted butter, whole milk, sour cream/yogurt, and vanilla extract until uniform.
- Gradually add the entire box of cake mix to the wet ingredients. Start the mixer on low speed, then increase to medium. Mix only until the last streaks of dry mix disappear, typically 1 to 2 minutes. Do not overmix.
- Scrape down the sides and divide the batter evenly between the two prepared tins. Place the tins on the middle rack and bake for 30–35 minutes.
- The cakes are done when a toothpick inserted into the center comes out clean. Allow the cakes to cool in the tins on a wire rack for 10–15 minutes, then gently turn them out, remove the parchment paper, and let them cool completely (at least 60 minutes) before frosting.
- Cream the softened butter in a stand mixer until light and fluffy (about 3 minutes). Slowly add the sifted icing sugar one cup at a time, mixing on low speed, then increasing speed to medium.
- Add vanilla extract and 2 tablespoons of milk/cream. Adjust consistency with the remaining milk/cream, adding 1 teaspoon at a time until the desired fluffy spreading consistency is reached. Frost the cooled cakes, layer, and decorate before serving.