Heat oven to 350°F. Line 13x9-inch pan with 18x18-inch piece heavy-duty foil so foil extends over long sides of pan. Spray foil with cooking spray. In large bowl, beat 1/2 cup butter, the brown sugar and granulated sugar with electric mixer on medium speed 2 minutes or until creamy. Beat in vanilla and eggs.
On medium speed, beat in flour, baking powder and salt until mixed. With spoon, stir in chopped pecans. Spoon and spread evenly in pan.
Bake 20 to 30 minutes or until toothpick inserted in center comes out clean. Cool completely, about 1 hour.
In small bowl with wooden spoon, beat powdered sugar, 1/4 cup butter and enough milk for desired spreading consistency. Stir in food color. Lift foil with brownies from pan. With sharp knife, trim 1/4 inch from long sides. Cut brownies into 3 lengthwise strips.
With metal spatula, lift 1 strip to cutting surface. On 1 long side, mark at 2-inch intervals with a small cut or with toothpicks. On remaining long side, make a mark 1 inch from short side. Make marks every 2 inches. With sharp knife, cut into triangles, using small cuts as guidelines. Frost triangles with 1/3 of frosting. Sprinkle with candy decors. Pull apart slightly before frosting is set. In center of bottom edge of each “tree”, cut small slit with point of sharp knife. Insert pecan half into slit to resemble trunk of tree. (There will be a small piece at each end of each row that will not be a complete tree.) Repeat with remaining 2 rows of brownies and frosting.