Whisk the flour, baking powder, and salt together in a medium bowl. Set aside.

In a large bowl using a handheld or a stand mixer fitted with a paddle attachment, beat the butter and sugar together on high speed until completely smooth and creamy, about 3 minutes. (Here’s a helpful tutorial if you need guidance on how to cream butter and sugar.) Add the egg and vanilla and beat on high speed until combined, about 1 minute. Scrape down the sides and up the bottom of the bowl and beat again as needed to combine.

Add the dry ingredients to the wet ingredients and mix on low until combined. Dough will be a bit soft. If the dough seems too soft and sticky for rolling, add 1 more Tablespoon of flour.

Divide the dough into 2 equal parts. Place each portion onto a piece of lightly floured parchment paper or a lightly floured silicone baking mat. With a lightly floured rolling pin, roll the dough out to about 1/4-inch thickness or slightly thinner (anywhere between 1/8 – 1/4 inch is fine). Use more flour if the dough seems too sticky. The rolled-out dough can be any shape, as long as it is evenly thick.

Lightly dust one of the rolled-out doughs with flour. Place a piece of parchment on top. (This prevents sticking.) Place the 2nd rolled-out dough on top. Cover with plastic wrap or aluminum foil, then refrigerate for at least 1-2 hours and up to 2 days.

As the cookie dough chills, begin crushing the candies. The best way to crush the hard candies is to separate them by color, place each color in plastic bags, and crush them with a rolling pin or meat mallet. Don’t crush the candies into a fine powder—crush them into fine pebble-sized bits. The crushed candies will pierce the bag no matter how careful you are. Some candy dust will escape, so just know that this step is messy. If you don’t want to use plastic, use a paper bag or paper plate. (A bag is preferred so candy doesn’t go flying everywhere.) Set crushed candy aside.

Once cookie dough has chilled, preheat oven to 350°F (177°C). Line 2-3 large baking sheets with parchment paper or silicone baking mats. Carefully remove the top dough piece from the refrigerator. If it’s sticking to the bottom, run your hand under it to help remove it—see me do this in the video above. Using your large cookie cutter, cut the dough into shapes. Using your small cookie cutter, cut out the center shape. (You don’t need the center piece—re-roll it!) Re-roll all the remaining dough and continue cutting until all is used. Repeat with 2nd piece of dough. Note: It doesn’t seem like a lot of dough, but you get a lot of cookies from the dough scraps you re-roll.

Carefully arrange cookies on baking sheets 3 inches apart. If your cookies are intricately shaped, use two hands to transfer them to the lined baking sheet.

If using an egg white, beat it lightly with a fork or small whisk for 10 seconds. It’s easier to brush onto tops of cookies if it’s beaten first. Brush tops of shaped cookies with egg white or water. Sprinkle coarse sugar/sprinkles on top.

Fill the cut-out centers about 3/4 full with crushed candies. The candy pieces melt and expand, so you don’t need to over-fill. I use and recommend 1 color per cookie, but feel free to mix for a spotty multicolor look. The candies are sticky, so wipe off your fingers as needed.

Bake for 12-13 minutes or until candies have melted and the cookies are lightly browned around the edges. If your oven has hot spots, rotate the baking sheet halfway through bake time. Allow cookies to cool on the baking sheet until the melted candy center sets, about 15 minutes. Once melted candy center has set, transfer to a wire rack to cool completely.

Cover and store cookies at room temperature for up to 1 week. For longer storage, cover and refrigerate for up to 10 days.