Preheat oven to 350 F. Lightly spray or butter a 8x8-in. baking dish with non-stick cooking spray or butter. I also line my pan with a strip of aluminum foil to help me lift them from the pan.
In a mixing bowl, combine the flour, sugar, cocoa, baking powder, and salt. In another bowl, combine the oil, eggs and vanilla. Add the wet ingredients and nuts (optional) to the dry ingredients. Mix just until combined. Spread into the prepared baking pan.
Bake for 35-40 minutes or until a toothpick inserted near the center tests clean. It should be just set in the center as they will continue to bake for the next few minutes as they cool.
Once brownies are cool, cut into Christmas tree-shaped triangles. I do this by first cutting the square into 3 equally sized rows (about 2-1/2 inches tall). Then I cut each row into triangles by angling the knife at about a 45 degree angle (see photo). Repeat with the remaining two rows.
Use as a piping bag, pipe frosting onto each brownie. The easiest way is to turn the brownie so the point of the tree faces you and the long side is at the top. Starting at the long side now at the "top", pipe frosting back and forth going down to the brownie tip. (Where the tree topper would go).
Use a variety of sprinkles, crushed peppermint candy canes, or small candies to decorate the trees. Store refrigerated in a tightly covered container.
Notes
Recipe makes between 18 to 30 cookies depending on the size you cut them.