Loaded Christmas Cookie Bars

Preheat oven to 350°F.
Line a 9×13 metal baking pan with aluminum foil, using metal binder clips to keep foil in place. Spray sides and bottom of pan generously with cooking spray. Set aside.
In a medium bowl, combine the flour, baking powder, baking soda, and salt. Set aside.
In the large bowl of an electric mixer fitted with the paddle attachment, beat the butter, peanut butter, brown sugar, and granulated sugar on medium-high speed until very well combined, about 2 minutes.
Scrape down the sides and bottom of the bowl. Add the eggs, vanilla, and milk, and beat until combined.
On low speed, gradually add the flour mixture and beat until just combined. The dough will be thick.
Add the peanut butter chips, M&Ms, and chocolate chips and mix until just combined. You may need to do this by hand with a spatula if your mixer isn’t strong enough.
Remove the dough to the prepared pan, pressing into the pan evenly.
Sprinkle with the reserved M&Ms and chocolate chips.
Bake for about 30 minutes, or until the center is golden brown and just slightly jiggly.
Sprinkle with flaky sea salt.
Place pan on a wire rack and let cool completely before cutting into squares and serving. Store bars in an airtight container at room temperature for up to 5 days.