Step 1Bake cake according to package directions for 2 (8-inch) round cake pans. Let cool. Cut each cake in half horizontally to make 4 layers.Step 2In a medium saucepan over medium heat, combine sweetened condensed milk, butter, and vanilla. Cook about 5 minutes, or until thickened, stirring occasionally. Stir in pecans and coconut.Step 3Place one cake layer on a serving platter. Spread 1/4 of the coconut mixture on top. Repeat with remaining layers and garnish with pecan halves. Refrigerate until ready to serve.