Place chunks of butter in a saucepan over medium heat, stir occasionally.
Once the butter has melted, it will begin to foam. Keep cooking and stirring. The foam will subside, and the butter will start to sizzle and bubble.
The milk solids in the butter will start to brown. You’ll notice a nutty aroma and see the butter turn golden brown. Keep a close eye on it to prevent burning and continue stirring.
When the butter has turned a deep golden brown and you see browned bits (milk solids) at the bottom of the pan, remove it from heat immediately. This takes approximately 4 - 6 minutes.
Pour the browned butter into a heat-resistant bowl. Let it cool at room temperature for 30 minutes before proceeding with the rest of the recipe.
Prepare Baking Pan
Lightly grease an 8" x 8" square cake pan with baking spray or melted butter.
Make two pieces of parchment paper that are 8" x 14".
Place one sheet over the pan and press it into the bottom and sides, creasing the paper where the edges of the pan meet.
Lay the second sheet crossed over the first sheet and extending up the sides of the pan.
Make sure it hangs over two opposite sides of the pan. These will act as handles to lift out the blondies.
Blondies
Preheat oven to 350℉
Combine flour and salt in a bowl. Sift flour if necessary to remove lumps. Set bowl aside.
Add brown sugar to bowl with cooled brown butter. Whisk until smooth.
Add eggs to bowl. Whisk until smooth.
Add flour and salt to bowl. Stir until smooth and no dry pieces of flour remain.
Add M&M's to mixing bowl. Stir until evenly distributed.
Add blondie dough to prepared baking pan. Use your hands or an offset spatula to smooth dough evenly.
Bake for 25 - 30 minutes. Top will be evenly browned and slightly risen.
Cool for one hour. Pull up on parchment to remove blondies from pan. Cut to desired sized pieces.
Decorating
Place softened butter in a mixing bowl. Mix on medium speed until butter is smooth (2 - 3 minutes).
Add powdered sugar and water to mixing bowl. Start mixing on low speed and slowly increase speed to medium-high. Mix until smooth and creamy (2 - 3 minutes).
Remove ⅓ cup of buttercream and place in a separate bowl. Add a few drops of yellow food coloring and stir until combined. Transfer to a pastry bag fitted with a small star tip.
Add a few drops of green food coloring to remainder of buttercream and stir until combined. Continue adding more green drops until you achieve your desired color. Transfer to a pastry bag fitted with a small round tip.
For the Christmas trees, pipe a squiggle of green icing down slices of blondies (start with a small squiggle at the top and increase size of squiggle as you go down).