Preheat oven to 350°F. Line two baking sheets with silicone mats or parchment paper.
In a bowl, whisk together flour, baking powder, baking soda and salt. Set aside.
Using a hand mixer or stand mixer fitted with the paddle attachment, beat butter and both sugars for 2 minutes, until light and fluffy. Add egg and vanilla extract, mixing until combined, scraping the sides of the bowl as needed. On low speed, add in flour mixture and mix until just combined.
Gently stir in M&M's/chocolate chips - I usually add about ¾ cup into the batter, the rest get pressed on top of the cookies before baking.
Scoop balls of dough, between 1.5 and 2.5 tablespoon portions (a little smaller or a little larger than a golf ball). Drop dough onto prepared baking sheets leaving an inch or two for spreading. I like to mound the dough balls so that they're higher rather than wider (for thicker cookies). Using the remaining M&M's/chocolate chips, nestle several (5 or so) onto the top of each mound of dough. Keep in mind that the cookies will spread out while baking, so be generous.
Bake for 8-11 minutes (see note), until the edges of the cookies are set but the center is still slightly underdone, this will create a soft cookie. For a crispier cookie, continue baking for another minute or two. See note about getting perfectly round cookies.
Place the baking sheets on wire racks and allow the cookies to cool completely. They will firm up as they cool.