Heat your oven to 350°F and line a square baking dish with parchment paper.
In a large bowl, whisk together the melted butter, brown sugar, and granulated sugar until smooth. Add in the eggs and vanilla, and mix until combined.
In a separate bowl, whisk together the flour, baking powder, and salt.
Slowly add the dry ingredients to the wet mixture, stirring gently. Once the dough thickens, switch to a spatula and fold until just combined. Be careful not to overmix!
Fold in the Easter M&Ms, reserving a few to sprinkle on top for extra color.
Spread the dough into your prepared baking dish and bake for 35-40 minutes, until the top is golden and puffy. The edges should be slightly crisp, but the center should still be soft.
Let them cool completely before slicing into squares.