Melt butter in medium or large saucepan with heavy bottom. Continue cooking until color turns from yellow to light brown. Remove from heat immediately.
Add sugars and stir thoroughly. Mixture will resemble wet sand. Let cool for 5 minutes.
Add vanilla and eggs. Stir to thoroughly combine.
Add flour, baking powder, and salt. Stir until just combined.
Add white chocolate chips and 1 cup of the m&m's.
Spoon the dough into prepared baking dish and spread evenly.
Sprinkle remaining ¾ cup cup m&m's over the top.
Bake for 25-30 minutes until edges are lightly browned.