Line a sheet tray with parchment paper, set aside. In a large bowl, stir together the flour, cornstarch, baking soda, baking powder, and salt until combined, set aside.
In a large bowl with an electric hand mixer or in the body of a stand mixer with the paddle attachment, cream the butter with the sugar for 2 minutes. Add the egg and mix it in until combined. Add the peanut butter and vanilla extract, mix it in until combined, scrape down the sides as needed.
Add the dry mixture to the wet and stir it in until combined. Add violet-colored gel food coloring until you achieve the color you want. I like to give a few good stirs by hand to make sure the color is evenly distributed.
Using a 1 tablespoon cookie scoop (or you can eyeball it), place scoops of the dough not touching but closer together on the prepared sheet tray. Cover with plastic wrap and place in the fridge for at least 2 hours to chill, even best overnight if you have the time.
Preheat the oven to 350°F. Line sheet trays with parchment paper, set aside.
Place the sprinkles or sanding sugar into a small bowl. Roll the cookie dough into a ball, then roll it in the sprinkles/sugar. Place the cookies 2 inches apart on the sheet trays. Bake for 9–11 minutes until puffed up and start to crack.
Let the cookies cool for 5 minutes on the sheet tray. Place a hug in the center of the cookies, pressing down until it sticks. Place the try directly into the freezer for 10 minutes to prevent the chocolate from melting. Transfer to a wire rack while you bake the rest of the cookies.