Preheat oven to 350 F. Line and grease a 9-inch round cake pan, 8-inch round cake pan and a 7-inch square cake pan with parchment paper. In a large bowl, whisk together flour, sugar, salt, baking powder and baking soda.
Add in eggs, milk, vegetable oil and vanilla extract and whisk until smooth. Divide the cake batter between the three cake pans. Bake for 30 minutes, let cool completely.
For the buttercream frosting, beat softened butter with powdered sugar, milk and vanilla extract until smooth. Divide a small portion of the frosting into a small bowl, mix with the cocoa powder until smooth.
To assemble the cake, cut the square cake into one long strip and a rectangle for the hat. Place the rectangle over the strip. Place the smaller cake layer under the hat followed by the large cake under. Frost the round cake layers with the vanilla frosting, and the top hat with the chocolate frosting. Decorate cake with m&ms for the eyes and scarf, the mini chocolate chips for the mouth and the pretzel sticks for the arms.
