Heat a large sauté pan over medium-high heat. Add your butter and melt fully, then cook until bubbly and golden brown.
Remove from heat and let cool slightly. Add heavy cream and vanilla and stir to combine.
Add your white and brown sugars and whisk into a thick slurry. Then, add your eggs, one at a time, and mix until combined.
In a separate bowl sift together AP flour, bread flour, baking powder, baking soda, and kosher salt.
Add your flour mixture to your brown butter, sugar, and egg mixture. Stir with a spatula until fully incorporated and thick.
Add your M&M's and mix just until combined. Cover your pan and place in the fridge to chill for 1 hour.
Preheat oven to 375°F. Line two baking sheets with parchment paper.
Scoop 10 equal balls of cookie dough onto your sheets, spacing at least 2 inches apart, 5 on each sheet.
Bake for 11 - 14 minutes until edges are golden brown and middles are beginning to take on color. Remove from oven and let cool on pan.
Top with a touch of flaky sea salt and serve warm.