Preheat oven to 350 degrees F. Coat the inside of a square baking pan with non-stick cooking spray, line with parchment paper, and then spray again. Set aside.
Put the chocolate and butter in a microwave-safe bowl. Heat in the microwave at a low setting for 1 minute and then stir. Repeat heating at 30-second intervals and then stirring until melted and smooth. Let cool while you prepare the other ingredients.
In a large bowl, beat together eggs, sugar, and vanilla.
In another bowl, sift together flour, baking powder, salt, and cinnamon.
Pour the melted chocolate into the egg mixture. Mix to combine.
Add the dry ingredients to the wet ingredients and mix until completely incorporated. Pour into your prepared baking pan.
Bake for 20 minutes, or until the edges are set and the middle is still a little gooey. Remove from oven and cool in pan until room temperature.
Use the parchment paper to lift the brownies from the pan. Cut the brownies into 8 triangles (see image). Place a biscuit stick into the bottom of each tree for the trunk.
Pour the powdered sugar into a bowl and add 2 teaspoons of water. Whisk to combine. Add another teaspoon of water if needed. The icing should drizzle, but hold its shape.
Pipe the icing over the brownie trees and add sprinkles while the icing is still wet. Let rest until icing set, and then enjoy!