Heat the oven to 375 F and line a cookie sheet with parchment paper or a silpat mat. The lined pan helps the cookies bake evenly and release cleanly once the edges turn golden.

Whisk together the flour, baking powder, baking soda, and salt in a medium bowl, then set it nearby. An even dry mix prevents salty spots and helps the leaveners distribute through the dough.

Beat the softened butter with the granulated sugar and brown sugar in a large bowl or stand mixer until the mixture turns pale and fluffy, scraping the bowl as needed. After 2 to 3 minutes it should look lighter in color and feel airy rather than dense or greasy.

Mix in the egg and vanilla until the dough base looks smooth and fully blended. You should not see streaks of egg, and the batter should smell sweet and warm from the vanilla.

Add the dry ingredients gradually and mix just until the flour disappears. Stop as soon as the dough comes together so the cookies stay tender instead of turning heavy.

Stir in the mini chocolate chips and m&m's until they are evenly scattered through the dough. The finished dough should feel thick and soft, and a pressed scoop should resist your fingertip a little instead of collapsing flat.

Scoop the dough into large balls, about 3 Tablespoons each, and place them on the prepared baking sheet about 2 inches apart. Leave the tops rounded rather than flattened so the cookies keep more height as they bake.

Bake for 8 to 10 minutes, pulling the pan when the edges are light golden brown and the centers still look soft and slightly underdone. The aroma should be buttery with a clear vanilla note, and the middles should look set at the edges but not fully browned.

Let the cookies rest on the baking sheet for 1 minute, then move them to a cooling rack to finish cooling. That short rest helps them firm up enough to lift without losing their soft centers.