In the bowl of an electric stand mixer, combine the butter and sugar. Cream on high speed until the mixture goes from yellow to white, about 5 minutes.
Mix in the eggs, one at a time, followed by the vanilla.
In a separate bowl, stir together the flour, cream of tartar, baking soda, salt, and milk powder. Add slowly to the creamed mixture. Mix on low speed only until the flour disappears.
Turn the dough out onto a piece of parchment paper and form it into a cylinder with your hands. Divide it as evenly as possible into three pieces.
Set aside one portion and put it into the container you'll be using to chill your cookie dough. This is your vanilla part.
Place another portion back into the mixer bowl. Add the Strawberry Nesquik (or freeze dried berries) and food coloring. Mix until pink. Fold in the berry pieces, if using. Add to the container with the vanilla dough.
Add the last portion of dough to the mixer, and mix in the cocoa powder. Add to the container of dough.
Chill for at least an hour, but preferably overnight.
Preheat oven to 350 F.
Scoop across all three doughs using a 1/3 ounce scoop (about the size of a ping pong ball). Aim for a fairly equal mix of all three and use your hand to pack the dough tightly into the scoop.
Space evenly on a parchment-lined baking sheet with a couple of inches between each scoop.
Roll in sprinkles, if using.
Bake until cookies spread nicely, and the vanilla parts are golden around the edges, about 15 minutes.