In a medium mixing bowl, combine your flour, baking powder, baking soda, and salt. Whisk to combine. Set aside to work on the wet ingredients.
In a large bowl, combine butter, granulated sugar, and brown sugar. Using an electric mixer fitted with the paddle attachment, or by hand with a wooden spoon, cream together the butter and sugar until it turns a light pale color and you no longer see dry granules of sugar. Add your eggs in one at a time, whisking well after each addition. Add your vanilla and beat until everything is incorporated.
Add about half of your flour mixture to the butter/egg mix. Mix together until the flour is just combined. Add your milk and whisk until the milk is incorporated. Finish off by adding your remaining flour mixture, and whisk until everything is just combined. Don't over-mix!
Fold in your chocolate chips or chunks. Pour the batter into your prepared baking dish, and smooth into an even layer. Bake in your preheated 350°F oven for 25 to 30 minutes, or until a skewer inserted into the center comes out clean. Cool to room temperature before slicing and serving!