Preheat the oven to 350° Fahrenheit.
Line an 8x8 baking pan with parchment paper. I find it easiest to grease the pan, then line it so it sticks right in.
In a mixing bowl or bowl of a stand mixer add the softened butter (I usually just microwave for about 30 seconds) and both sugars. Cream together for 1 minute.
Add in the milk and the vanilla.
Add in all dry ingredients aside from the sprinkles and fold together. Be sure to spoon and level the flour to measure, don't pack them in. The dough should seem pretty similar to regular cookie dough.
Fold in the sprinkles.
Transfer to your prepared pan and press down flat with a rubber spatula. Sprinkle on top 1-2 teaspoons of sprinkles.
Bake for 20-25 minutes or until the edges are just golden brown. You want to be careful about both over and under baking here. Over-baking will result in a dry texture and under-baking they will be too gooey on the inside.
Once done, remove from the oven and let cool on a wire rack. Remove from the pan once cool and slice into 16 squares. If you want them extra sweet, you can frost with the frosting recipe in the notes section!