The confetti cookie combines the technique of a snickerdoodle (cream of tartar makes all the difference in telling an average cookie apart from a snickerdoodle-inspired one) with the flavors of Funfetti cake mix.
Combine the butter, sugar, and glucose in the bowl of a stand mixer fitted with the paddle attachment and cream together on medium-high for 2 to 3 minutes. Scrape down the sides of the bowl, add the eggs and vanilla, and beat for 7 to 8 minutes.
Reduce the mixer speed to low and add the flour, milk powder, cream of tartar, baking soda, salt, and rainbow sprinkles. Mix just until the dough comes together, no longer than 1 minute. (Do not walk away during this step, or you will risk overmixing the dough.) Scrape down the sides of the bowl with a spatula.
Still on low speed, add the birthday cake crumbs and mix just until incorporated, about 30 seconds.
Using a 2 ¾-ounce ice cream scoop (or a 1/3-cup measure), portion out the dough onto a parchment-lined sheet pan. Pat the tops of the cookie dough domes flat. Wrap the sheet pan tightly in plastic wrap and refrigerate for at least 1 hour, or up to 1 week. Do not bake your cookies from room temperature — they will not bake properly.
Heat the oven to 375 degrees. Arrange the chilled dough a minimum of 4 inches apartment on parchment-or Silpat- lined sheet pans. Bake for 18 minutes. The cookies will puff, crackle, and spread. After 18 minutes, they should be very lightly browned on the edges (golden brown on the bottom). The centers wil show just the beginning signs of color. Leave the cookies in the oven for an additional minute or so if the colors don’t match and the cookies still seem pale and doughy on the surface.
Cool the cookies completely on the sheet pans before transferring to a plate or airtight container for storage. At room temp, the cookies will keep for 5 days; in the freezer, they will keep for 1 month.
Preheat the oven to 300 degrees.
Combine the sugars, flour, baking powder, salt, and sprinkles in the bowl of a stand mixer fitted with the paddle attachment and mix on low speed until well combined.
Add the oil and vanilla and paddle again to distribute. The wet ingredients will act as glue to help the dry ingredients form small clusters; continue paddling until that happens.
Let the crumbs cool completely before using in a recipe or scarfing by the handful. Stored in an airtight container, the crumbs will keep fresh for 1 week at room temperature or 1 month in the fridge or freezer.