Preheat oven to 350°F Butter and flour three 9-inch-diameter cake pans. Line the bottoms with parchment paper, then butter and flour the parchment paper.
In large bowl, cream together butter and sugar with electric mixer. Add peanut butter and vanilla and cream well. Add eggs one at a time, mixing well after each.
In separate bowl, whisk together dry ingredients.
Add dry ingredients to peanut butter mixture alternately with sour cream and milk. Mix on low speed just until blended.
Pour cake batter into prepared pans (approximately 2 cups per pan), spreading evenly in pans.
Bake for 20-30 minutes or until tester inserted into center of cake comes out clean. Do not over bake. Remove from oven and cool cakes in pans on rack 10 minutes.
Run small sharp knife around pan sides to loosen cakes. Turn cakes out onto racks and cool completely.
Peanut Butter Filling:.
Using an electric mixer, beat together butter, peanut butter and vanilla in large bowl until light and fluffy.
Beat in the heavy cream until combined.
Add 1/2 cup powdered sugar, mix well. Add in the remaining powdered sugar, 1/2 cup at a time, mixing well after each addition.
Milk Chocolate Ganache:.
Heat the heavy cream, butter, and sugar in a 2 1/2-quart saucepan over medium-high heat. When hot, stir to dissolve the sugar. Bring the mixture to a boil. Place the chocolate in a stainless steel bowl. Pour the boiling cream over the chocolate and allow to stand for 5 minutes. Stir until smooth. Allow to cool to room temperature. Keep the ganache at room temperature until needed. Yield: 2 cups.
To Assemble Cake:.
Place one layer of cake on a serving platter. Spread half of the peanut butter filling on top of the cake.
Coarsely chop 20 Reeses miniature peanut butter cups. Sprinkle 1/2 of chopped peanut butter cups over peanut butter filling on top of first layer.
Top with another cake layer. Spread remaining peanut butter filling on top of the second cake layer and then top with remaining chopped peanut butter cups. Top with final cake layer.
Use a cake spatula to smooth and fill in the sides of the cake with 2 to 3 tablespoons of rthe oom-temperature ganache. Evenly spread the remaining amount of this ganache over the top and sides of the cake. Refrigerate the cake for 1 hour.
Drizzle with peanut butter sauce, optional.
Coarsely chop remaining 10 Reeses miniature peanut butter cups and sprinkle over top of cake.