Preheat oven to 350F degrees. Line the inside of a 12-inch cake pan with parchment paper and spray the inside with nonstick spray.
Add the brownie mix plus required ingredients to a stand mixer and beat until combined. Add peanut butter chips and fold in with a spatula.
Add batter to prepared cake pan and bake 30-35 minutes. Let cool completely before removing from pan.
Once cool, frost an even layer of white buttercream all over the top of the cake.
Add the remaining white frosting to a bowl and add a couple drops of turquoise gel food coloring. Fold together with a spatula until combined and transfer to a pastry bag fitted with #825 star tip. Set aside.
Transfer the blue decorating icing to a small pastry bag fitted with a (#6) round tip. Pipe 7 vertical lines at the top of the brownie, alternating long and short lines. Place 2 silver dragees underneath the shorter icing lines.
Pipe the turquoise frosting underneath the blue vertical lines in a horizontal squiggle pattern.
Place a horizontal line of purple gum drops directly under the turquoise frosting.
Use the green decorating gel to pipe two horizontal lines across the brownie underneath the purple gumdrops. Pipe vertical lines in between the horizontal lines so that it looks like a sideways ladder.
Transfer the pink decorating icing to a pastry bag fitted with a #825 star tip. Pipe 6 pink stars underneath the green decorating icing.
Mix the sprinkles together and scatter over white frosting underneath the pink frosting stars.
Insert the candy cane into the top of the ornament. Use kitchen shears to make a vertical cut halfway into 2 marshmallows. Place those on both sides, opening up the slits, to secure the candy cane inside so that it is not exposed. Add one more marshmallow on each side so there are 4 marshmallows at the top.