To Prep: Preheat the oven to 375 degrees F and line a baking sheet with parchment paper.
Make the Dough: In the bowl of a stand mixer (or large bowl with an electric mixer), cream the butter for about 1 minute. Add sugar and mix until well combined and fluffy. Add egg and vanilla and mix thoroughly. Add in the flour and salt and mix until it just comes together. Scoop and roll dough into balls about a teaspoon in size.
Roll each cookie dough ball in the colored sugar and place on baking sheet.
Using the back side of a cookie scoop or melon baller, press each cookie dough ball down to make an indentation, or "thumbprint" in the center.
Bake for 10-12 minutes until the cookies just start to turn golden brown. Allow to cool completely and then add a small scoop of jelly in the middle of each.
Notes
Nutrition information is provided as an estimate only and will vary based on exact ingredients used and serving size.