Combine the egg whites and sugar in a stand mixer bowl and whisk to combine. Place bowl over simmering water while whisking regularly until the mixture reaches 140 degrees F on an instant-read thermometer and the sugar is completely dissolved. Remove bowl from heat and whip the mixture at medium to high speed until the meringue has cooled almost completely.
Slowly add the room temperature butter piece by piece until the buttercream takes on a cottage cheese consistency. Add the melted cooled chocolates, spices, and vanilla until incorporated and smooth.
Preheat the oven to 375 degrees F.
In a medium bowl combine the flour, baking powder, and salt. Set aside.
In the large bowl combine the butter and the sugar. Using an electric mixer beat the butter and sugar until light and fluffy, about 3 minutes. Add the eggs, 1 at a time, beating until incorporated. Add the milk, buttermilk. Beat to combine. Stir in the dry ingredients.
Line 2 baking sheets with parchment paper. Spoon the dough, about 2 tablespoons of dough onto the baking sheets. Make sure the dough is seperated on the baking sheet. Bake for 15 minutes, until slightly golden at the edges. Remove from the oven and let the cookies rest on the baking sheet for 20 minutes. Frost cookies. Decorate with sprinkles.
Makes 3 dz cookies