Pre-heat oven to 350 F. Butter and flour a 8x8 pan and set aside.
In a large bowl, whisk together melted butter, brown sugar and granulated sugar until smooth. Add egg and clear imitation vanilla extract and whisk well. Add flour, salt and 3 tablespoons of sprinkles. Mix together until well combined.Evenly spread batter in the greased pan and sprinkle the rest of the sprinkles on top.
Bake for 20-25 minutes. You want the edges to be golden brown and middle to be a little gooey since it will continue cooking in the hot pan when it is cooling. Cool completely and enjoy!