Preheat the oven to 325 degrees F. Grease and line two 6 inch cake pans with parchment paper.

In a medium bowl, whisk together the flour, cocoa powders, baking powder, and salt. Set aside.

In the bowl of a stand mixer fitted with the paddle attachment, cream together the butter and sugar for 2-3 minutes until light & fluffy.

Mixing on low speed, add the eggs one at a time. Scrape down the bowl and mix in the vanilla and peppermint extract.

Add half of the dry ingredients and mix until just combined. Mix in the sour cream and oil, then the remaining dry ingredients until just combined. Scrape down the bowl to ensure everything is evenly incorporated.

Divide the batter between the prepared cake pans and smooth them out into even layers with an offset spatula.

Bake the cakes for about 35-40 minutes or until the edges begin to pull away from the sides of the pan and a toothpick inserted comes out clean.

Turn the cakes out onto a wire rack to cool completely.

In a small heat-proof bowl, add the white chocolate and place it over a pot of simmering water. Stir often until fully melted. Remove the bowl from heat and set aside.

Place the egg yolks and 24g (2 tbsp) of granulated sugar in a medium bowl or a bowl of a stand mixer fitted with a whisk attachment. Whisk on high speed until thick and foamy (about 3 minutes) while you heat the sugar and water.

Heat the remaining 100g (½ cup) of granulated sugar and water in a small saucepan over low heat, stirring constantly until the sugar dissolves. Increase the heat to medium and bring to a boil, or until it reaches 240 degrees F.

While the egg yolks are whisking on medium speed, slowly add the sugar syrup into the bowl. Continue mixing until the mixture has cooled down.

Add the butter to the mixture a few cubes at a time, allowing them to fully incorporate before adding more.

Lastly, add the cooled melted white chocolate and vanilla. Continue mixing on low speed until the buttercream is completely smooth.