Preheat oven to 350 degrees. Line a baking sheet with parchment paper or a silicone baking mat and set aside.
In a large mixing bowl, use a hand-held mixer to beat together the butter and sugar until well incorporated.
Next, beat in the egg yolks and vanilla extract.
Lastly, mix in the flour. The mixture will be crumbly at first and might not look like a cookie dough, but keep beating on medium speed and a cookie dough will form. This is not an overly wet dough, so don't be alarmed.
Portion two teaspoons of the cookie dough and roll into a ball. Roll the ball into the sanding sugar or sprinkles.
Place the ball onto the prepared baking sheet. Gently, but firmly, press one candy melt into the center of the cookie, flattening the cookie as you do so.
Bake for 12 minutes. Remove from oven and allow cookies to cool for 3-5 minutes before transferring them to a wire cooling rack to finish cooling.