Preheat oven at 350°F. Grease three 6-inch cake pans.
In a large bowl, whisk together all the dry ingredients. Set aside.
In a mixer with a paddle attachment, beat the butter and brown sugar together until light and fluffy. Add the eggs, one at a time and mix until well incorporated.
Add in the dry ingredients and start mixing on slow. Gradually add the milk and sour cream.
Divide batter evenly into three cake pans.
Bake for 26 - 29 minutes until the cake springs back when you press the top gently. Let cakes cool before removing them from pan to completely cool.
Sour Cream Chocolate Frosting
Melt chocolate and butter together on top of a double broiler. Set aside and let mixture cool.
Once chocolate mixture has cooled, transfer mixture to the bowl if a mixer and combine with sour cream, vanilla, and salt.
Gradually add the icing sugar and cocoa powder and mix on low until incorporated. Turn mixer on high and mix until frosting is fluffy.
Simple Vanilla Buttercream
In the bowl of a mixer, beat butter until pale and fluffy. Add vanilla.
Gradually add the icing sugar and mix until incorporated. Add milk, one tablespoon at a time.
Mix on high until buttercream is at desired consistency.
Separate buttercream into bowls to colour with food colouring.