Line two baking sheets with parchment paper or silicone baking mats.
Mix dry ingredients together: 2 cups flour, 1/4 teaspoon baking powder, 3/4 teaspoon baking soda, 1/4 teaspoon salt.
Using a stand mixer fit with the paddle attachment, or a hand mixer, cream together the 1/2 cup butter, 1/2 cup granulated sugar, and 1/2 cup brown sugar.
Mix in the egg for about 30 seconds. Mix in the 1 teaspoon vanilla and 2 tablespoons of milk.
Slowly add the dry ingredients to the wet ingredients. I like to do this in three batches to prevent flour from flying all over.
Scoop out roughly a golf ball size of dough and place onto baking sheet. I used an ice cream scoop and got about 9 cookies. You can make them smaller if you like.
Put the sheets of dough in the fridge and let them chill for 20 minutes while you clean up. Preheat the oven to 350 F.
Bake the cookies one sheet at a time, on the middle rack, for 18 minutes.
Once done, remove the cookies from the oven and let them rest for 5 minutes. The cookies will be very soft in the middle and look slightly underdone. Don't worry, they will finish cooking from the residual heat. Let them cool down all the way before eating. Or, enjoy an ooey-gooey one, if that's your thing.