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.
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.
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.
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.
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.
