Preheat oven to 325
Prepare 3 nine inch cake pans with cooking spray. Wilton's Cake Release works great.
Add butter, sugars, vanilla, and eggs to large mixing bowl. Cream until well blended.
Sift flour, baking powder, and salt in a separate bowl.
Alternately add flour mixture and milk to sugar mixture. Use a medium setting.
Pour batter evenly into prepared pans. Bake for 25 minutes and check using a wooden toothpick. Continue baking for 5 minutes if needed.
Cool layers on baking racks.
Add butter and brown sugar into heavy sauce pan. Heat on medium until sugar is dissolved and butter is melted.
Add milk. Stir until milk is incorporated.
Bring to a rolling boil, stirring constantly.
Continue to heat for 4 minutes, stirring constantly.
Remove from heat and set aside. Allow to cool for 15 minutes.
Sift powdered sugar.
Add vanilla extract and powdered sugar to mixture. Beat on high for 2 minutes. Reduce speed and continue beating until the frosting has a thick consistency.
Allow frosting to cool for 30 minutes before frosting cake. If it's too thick, add a small amount of milk. If it's too thin, add a small amount of powdered sugar.
Add frosting between each layer and cover the top and sides generously with frosting.