Preheat oven to 350°F. Line three eight-inch pans with parchment rounds, and grease with non-stick baking spray.

Mix together all dry ingredients (flour, baking powder, sugar, and salt) in a stand mixer with a paddle until fully combined.

Mix chunks of room-temperature butter slowly into the dry mix, on a low speed. Continue to mix until no large chunks of butter remain, and the mixture becomes crumbly.

Pour in egg whites, and mix on low until just incorporated. Mix in the buttermilk in two installments, on a low speed.

Add in oil and vanilla and mix at a low speed until fully incorporated. Scrape down the sides of the bowl with a spatula, then beat on low speed for about 15 seconds. 

Mix the batter on a low speed, scraping the sides and bottom of the bowl part way through to evenly color the batter.

Divide batter evenly between the prepared cake pans.

Bake for 34-35 minutes (or until a skewer comes out with a few moist crumbs).

Allow to cool for 10 minutes, then run a small offset spatula around perimeter of the pan to separate the cake from the pan.

Place cake layers into the freezer for 45 minutes, to accelerate the cooling process.

Beat the butter on a medium speed for 30 seconds with a paddle attachment or hand mixer until smooth.

Mix in the vanilla extract and salt on a low speed.

Slowly add in the powdered sugar, 1 cup at a time. Alternate with small splashes of cream.

Mix on low for a couple extra minutes to make the frosting super smooth, and to get it to the desired consistency.

If the frosting is too thick, add in additional cream (1 Tablespoon at a time). If the frosting is too thin, add in more powdered sugar (quarter of a cup at a time).

Heat heavy cream in a heat proof bowl in the microwave for 45 seconds, until it's steaming and just beginning to bubble.

Gently pour white chocolate chips into the heavy cream, making sure they are fully covered with cream. Allow mixture to sit for 1 minute.

Stir slowly until the cream and white chocolate are combined. Some bits of chocolate may not be fully melted yet. Heat the mixture again for 20 seconds, and stir. Repeat as necessary until all the chocolate bits are fully melted and incorporated.

Once the ganache is smooth, mix in the gel food coloring until the ganache is evenly colored. If you plan to use a plastic bottle to add your drips, pour the mixture into a plastic bottle.

Allow the mixture to cool until the desired viscosity is reached (barely warm to the touch).

If it seems too thin or thick, you can add a touch more chocolate chips or heavy cream! The type of cream and white chocolate that you use can affect the consistency, so feel free to adjust as needed.

Stack and frost cake layers on a greaseproof cake board, using a dab of frosting to help stick the first cake layer to the board.

Add an even layer of buttercream between each cake layer with a large offset spatula.

Spread a thin coat of frosting around the the cake, to fully cover the cake layers.

Smooth using a bench scraper, then chill the cake in the fridge (20 minutes) or freezer (10 minutes) until the frosting is firm to the touch.

Add a second, thicker layer of frosting to the cake, and smooth using a bench scraper.

Chill the cake again in the fridge (20 minutes) or freezer (10 minutes) until the frosting is firm to the touch.

Make a test drip with your ganache to see if it's the right consistency (more detail on that in the post above).

Once it's the right consistency, add the drips to the chilled cake using a plastic squirt bottle or spoon. After covering the sides of the cake with ganache, chill the cake in the freezer for 5 minutes to help the drips set.

Add some of the remaining ganache on top of the cake and carefully spread it over the top of the chilled cake.

Then decorate as desired! I like to use the leftover buttercream to pipe swirls on top of the cake with a wilton 1M frosting tip.