I start by beating the butter and powdered sugar together in a large mixing bowl until the mixture turns light and fluffy. This usually takes about three minutes on medium speed. Getting plenty of air into the butter here helps the finished cookies stay tender instead of dense.
Next, I beat in the vanilla extract followed by the egg yolk, mixing just until everything is fully incorporated. The egg yolk adds richness without making the dough too wet, which keeps these Halloween inspired cookies sturdy enough to hold their shape.
I turn the mixer down to low and slowly add the flour and salt, mixing only until the dough just comes together. Overmixing at this stage is the easiest way to end up with tough cookies, so I stop as soon as I no longer see streaks of flour.
Using a spatula, I fold in the mini chocolate chips and most of the sprinkles, saving a small amount of sprinkles to press onto the tops later. This step is quick, and it is where the dough really starts to look festive for Halloween.
I divide the dough in half and roll each portion between sheets of parchment paper to about a quarter inch thick. Once rolled, I place both portions on a baking sheet, cover them, and chill for two hours until firm enough to cut cleanly.
After chilling, I lift the parchment paper onto the counter and cut the dough into Halloween shapes using my favorite cookie cutters. I place the cut shapes back on a lined baking sheet and chill them for another 20 minutes before baking.
I bake the cookies at 350°F for 10 to 12 minutes, just until the edges turn lightly golden. Once out of the oven, I let them rest on the baking sheet for 10 minutes before moving them to a wire rack to cool completely.