In a saucepan over medium heat, melt the butter and brown it for about 5-7 minutes. Remove from heat and let cool.
In a large bowl or kitchen aid, mix the browned butter, brown sugar, and granulated sugar until smooth.
Add the egg and vanilla extract, mixing well.
In a separate bowl, whisk together dry ingredients (flour, baking soda, baking powder, and salt).
Combine wet and dry ingredients, mixing until just combined.
Gently fold in m&ms and crushed pretzels.
Cover and refrigerate the dough for at least 30 minutes
Preheat oven to 350F and line a baking sheet with parchment paper.
Scoop tablespoon-sized balls of dough onto the baking sheet, spacing them out about 2 inches apart. Press a few extra m&ms and pretzel pieces onto each cookie.
Bake for 10-12 minutes, or until the edges are golden. Sprinkle with flaky sea salt (optional) as the cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely. Enjoy!