Line a 9x13-inch baking dish with parchment paper and set aside.
In the body of a stand mixer with the paddle attachment, add the cold butter, powdered sugar, and granulated sugar. Mix until well combined, scrape down the bowl.
Mix in the vanilla.
Add the flour in three batches, fully mixing it in before adding more, and scrape down the sides as needed.
Press the cookie dough evenly into the bottom of the prepared baking dish. Cover with plastic wrap and place in the fridge for at least 30 minutes, up to overnight.
Preheat the oven to 350°F. Line sheet trays with parchment paper and set aside.
Remove the cookie dough from the baking dish by using the parchment paper to help lift it out. Cut into ½ inch squares.
Arrange the cookie bites on the prepared sheet trays about 1 & ½ inches apart; they will spread a little.
Bake for 12-14 minutes until they very, very lightly start to brown along the edges. Allow to cool completely on the tray.
Once the cookies are cool, melt the different chocolates.
Drizzle ⅓ of them with the dark chocolate, ⅓ with the milk chocolate, and the last ⅓ with the white chocolate.
Allow them to sit until the chocolate has hardened and serve.
Notes
Please refer to my FAQ’s (Frequently Asked Questions) and ingredient list above for other substitutions or for the answers to the most common questions.
It's normal for the cookie dough to spread as they bake.