Beat 1 1/2 cups butter at medium speed with an electric mixer until creamy. Gradually add sugar, beating 7 minutes or until fluffy. Add eggs, 1 at a time, beating just until yellow disappears.
Combine flour and baking powder. Combine milk and vanilla. Gradually add alternately to butter mixture, beginning and ending with flour mixture. Beat at low speed just until blended after each addition. Spread batter into 2 greased and floured 9" cakepans.
Bake at 350° for 24 to 25 minutes or until a wooden pick inserted in center comes out clean. Cool in pans on wire racks 10 minutes. Remove from pans; cool on wire racks.
Break one-fourth of Old-Fashioned Peanut Brittle into 1 1/2" to 2" pieces. Place an additional one-fourth of brittle in a large zip-top freezer bag; crush into coarse crumbs using a mallet or rolling pin. Reserve remaining brittle for another use.
Spread Peanut Buttercream between layers and on top and sides of cake. Press coarsely crushed brittle onto sides of cake.
Place chocolate morsels, milk, and 1 tablespoon butter in a small glass bowl. Microwave on HIGH 1 minute. Stir until smooth. Dip ends of peanut brittle pieces in chocolate ganache; place on wax paper to harden. Drizzle remaining ganache over cake. Arrange dipped brittle on top of cake.
Make-Ahead Note: Prepare the peanut brittle and cake layers a day ahead, or make the entire cake a day ahead. Either way, add chocolate-dipped brittle just before serving.