Pre-heat oven to 325 degrees. Spray three 8-inch round cake pans with nonstick spray, line the bottom of the pans with parchment paper and spray again. Set aside
Using a double-boiler, melt the white chocolate and set aside.
Whisk together the dry ingredients and set aside.
Place the butter in the bowl of a stand mixer and beat on high until light in color. Add the sugar and continue to mix until fluffy, about 2 minutes.
Turn the mixer to medium-low, add in the whole eggs, egg yolks, vanilla, peppermint extract and the melted white chocolate. Stop the mixer and scrape down the sides of the bowl. Mix on medium-high for an additional 2 to 3 minutes.
With the mixer on low, alternately add in the dry ingredients and buttermilk in 3 batches, ending and starting with the dry ingredients. Mix until just combined.
Pour batter into prepared pans (about 16 ounces in each 8-inch pan) and bake until slightly golden in on top, 25 to 30 minutes. (A few moist crumbs should come out on a toothpick when testing the centers).
Let the cake layers cool about 10 minutes in the pans and then invert onto wire cooling racks.
When the cake layers are room temperature, level each layer (if needed), with a cake leveler. Wrap each layer with plastic wrap and chill until ready to use.