Then add the sugar gradually and beat until it is dissolved in the butter.
Separate the egg. Set the egg white aside.
Beat the egg yolk with a whisk or fork.
Add the egg yolk, vanilla flavoring and flour to the butter mixture, and mix by hand until it is a stiff batter.
Break off pieces of dough that are about 1 Tablespoon in size and shape into balls by gently rolling between your palms.
Dip each ball in slightly beaten egg white, then roll in chopped nuts or shredded coconut.
Place on a parchment lined baking sheet.
Make a dent in the center of each ball with a thimble, the back of a small measuring spoon or your finger.
Bake at 325°F for 15 minutes, or until the bottoms of the cookies are slightly browned.
Remove from the oven and immediately use the thimble to make the dents deeper.
Fill the dents with a ¼ teaspoon jelly while the cookies are still hot.
Cool on the pan for 10 minutes then remove to wire rack until completely cold.
Store in an air tight container for up to a week, or in the freezer for up to a month.
Notes
Use a jam or jelly that isn't too runny, otherwise you will have trouble keeping it on the cookie.
Sifting the flour before you measure it is very important to making shortbread that is fluffy and light.
Similarly, make sure you beat the butter and sugar well before adding other ingredients. The butter should be fluffy and the sugar should be dissolved into it. This is much easier to do if the butter is room temperature when you start. If you forget to take it out of the refrigerator, microwave it for a few seconds to soften it.
Mixing the flour in by hand is another important step to prevent the cookies from being too tough.