1. Preheat oven to 350°. To prepare cake, coat 2 (9-inch) round cake pans with baking spray. Line bottoms of pans with wax paper or parchment paper; coat paper with baking spray. Dust each pan with 1 teaspoon flour. Weigh or lightly spoon 9 ounces flour (about 2 1/4 cups) into dry measuring cups; level with a knife. Combine flour and next 3 ingredients (through salt); stir well.
2. Place 1/2 cup butter in a large bowl; beat with a mixer at medium speed until creamy. Gradually add granulated sugar, beating until light and fluffy. Add eggs, 1 at a time, beating well after each addition. Add flour mixture and buttermilk alternately to sugar mixture, beginning and ending with flour mixture. Beat in 2 teaspoons vanilla. Pour batter into prepared pans.
3. Bake at 350° for 25 minutes or until a wooden pick inserted in center comes out clean. Cool in pans 10 minutes on wire racks. Invert cake layers onto racks; cool completely. Discard wax paper.
4. To prepare frosting, melt 1/4 cup butter and chopped chocolate in a small heavy saucepan over low heat. Add low-fat milk, stirring with a whisk until smooth. Stir in 1 teaspoon vanilla. Remove from heat; transfer to a large mixing bowl, and cover surface of chocolate mixture with plastic wrap. Refrigerate until cooled completely and slightly thick, about 30 minutes.
5. Gradually add powdered sugar, beating with a mixer at medium speed until smooth and glossy. Place 1 cake layer on a plate; spread with 3/4 cup frosting. Top with remaining cake layer. Apply a thin layer of frosting to entire cake. Allow frosting to set in the refrigerator about 30 minutes. Spread remaining frosting over top and sides of cake. Sprinkle edge with candy sprinkles, if desired. Let cake stand until frosting is set.