Preheat oven to 350°F Line 13x9-inch baking pan with foil. Spray foil with no stick cooking spray. Mix flour, cocoa powder, cinnamon, baking powder and salt in medium bowl. Set aside.
Mix eggs, granulated sugar, sour cream, food coloring and vanilla in large bowl until well blended. Gradually stir in flour mixture until well mixed. Spread in prepared pan.
Bake 30 to 35 minutes or until toothpick inserted in center comes out with fudgy crumbs. Cool in pan on wire rack.
Spread Chocolate Buttercream Frosting evenly over cooled brownie. Lift from pan; remove foil. Cut into 24 squares.
Chocolate Buttercream Frosting: Mix 1 1/2 cups confectioners' sugar and 1/4 cup unsweetened cocoa powder in small bowl. Beat 1/4 cup (1/2 stick) butter, softened, in medium bowl with electric mixer on medium speed until light and fluffy. Add 1 teaspoon pure vanilla extract; mix well. Gradually add cocoa mixture alternately with 1 tablespoon milk, beating until light and fluffy. If frosting is too thick to spread, gradually beat in additional milk, 1 tablespoon at a time.