Mexican Hot Chocolate Buttercream:
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.
Cinnamon Vanilla Cake:
Cream the butter in a stand mixer until smooth and light. Add sugar and continue to mix until fully incorporated and fluffy.
Add 1 egg at a time, beating well after each addition. In a medium bowl sift together flour, baking powder, cinnamon, and salt.
Preheat oven to 350 degrees F.
Combine milk and vanilla. Add portions of the flour mixture and milk mixture to the electric mixer, alternating, starting and ending with flour. Pour evenly into 2 buttered and floured 9-inch cake pans.
Bake for 40 minutes or until golden - cake should pull away from the side and spring back to the touch.
Frost cake and decorate as desired.