Mix the butter and sugar together until it is light and fluffy. Blend in the eggs, one at a time.
In a separate bowl, whisk together the pumpkin and milk. Alternate adding the pumpkin mixture and dry ingredients to the egg mixture, beginning and ending with dry ingredients.
Grease and flour three 9" round cake pans. Pour the batter into the prepared pans, dividing equally.
Bake the cakes in the 350 degree oven for about 20 minutes or until the center of the cake bounces back when you touch it.
For the frosting:
Mix the butter and cream cheese together, scraping down the sides as needed, for about 2-3 minutes. With the mixer on low speed, add the powdered sugar, a little at a time, until it is thoroughly mixed. Add the vanilla and beat the frosting on medium speed for about 3 minutes.
Place one cake on a cake stand. Frost the top of the cake with a thick layer of frosting. Repeat with the other two cakes. Spread the frosting thin to achieve a naked cake look and garnish with blackberries. Store the cake in the refigerator until it's time to serve.