Preheat oven to 325F. Prepare 3 6-inch cake pans by spraying with nonstick spray and lining bottom with a parchment circle.

Cream butter and sugar together until light and fluffy (about 2-3 minutes). Scrape down sides of bowl.

Add one egg white at a time then add vanilla. Scrape down sides of bowl.

In a large bowl add flour, salt and baking powder. Whisk together.

Add 1/3 flour mixture and 1/2 buttermilk to butter mixture. Once incorporated add an additional 1/3 flour mixture and remaining buttermilk. Mix until incorporated. Scrape down sides of bowl. Add remaining flour. Only mix until all dry ingredients are mixed.

Remove bowl from stand mixer. Fold in sour cream.

Pour batter into 3 prepared 6-inch cake pans (about 12-13oz of batter in each pan).

Bake at 325F for about 30 minutes or until a toothpick poked in the center comes out clean.

Let cool in pans for 10-15 minutes then remove from pans and cool completely before assembling.

While cakes are baking, using a double boiler to heat egg whites, sugar and salt over simmering water. Make sure the bowl does not touch the simmering water. Stir mixture. Heat until mixture reaches 160F and sugar has dissolved (about 4-5 minutes). Test if sugar has dissolved by rubbing the mixture between two fingers. It should feel smooth with no grains.

When mixture is ready. Wipe bottom of bowl to remove any moisture then carefully pour mixture into a stand mixer bowl with a whisk attachment. Whip egg white mixture until it turns white, glossy and triples or quadruples in volume (10-15 minutes). The mixture should be very thick and fluffy. During this time the mixture will begin to cool. This is critical. Before adding the unsalted butter, the meringue mixture must be cool enough (70-75F) or the butter will melt.

Once the mixture has cooled to 70-75F, switch to the paddle attachment. Begin adding one tablespoon of room temperature unsalted butter. Make sure each piece has been incorporated into the meringue before adding that next piece. After all the butter has been added, add vanilla. Mix buttercream for an additional 7-10 minutes. It is ready when the buttercream no longer tastes like butter but rather like ice cream.

Split and color as desired.

In a microwave safe bowl, heat cream until it just begins to bubble. This will take about 45 seconds to a minute depending on your microwave.

Add chocolate. Make sure the chips are fully covered by the cream. Let the mixture sit for 1-3 minutes.

Stir. If small lumps remain, microwave for 25-20 seconds then stir again. If making pink and blue drip, split and color using gel food coloring.

Pour into two separate squeeze bottles. Test drip on side of bowl or in cup. When desired consistency is reached apply to chilled cake.