In a medium-sized mixing bowl, cream butter, brown sugar, and egg yolks until light and fluffy.
Mix in vanilla, flour, and salt until just combined.
Place tablespoon-sized balls of dough onto a cookie sheet lined with parchment or a Silpat, two inches apart from each other.
Bake for five minutes.
Streusel Topping Instructions
Using your hands, mix all streusel topping ingredients together in a small bowl until it comes together and forms crumbs.
Remove cookies from the oven and use your thumb, or any preferred finger, to make an indentation in the center of each cookie.
Fill the indentations with jam and top the jam with the streusel topping.
Return to the oven and bake for another 15 minutes or until cookies are golden brown.
Place cookies on cooling rack to cool.
You can fill the indentations with chocolate chips instead of jam. They will melt during the bake and you'll end up with awesome chocolate thumbprint cookies.Another chocolate variation is to use Hershey's Kisses like Baked by Rachel's snickerdoodle kiss cookies. For that variation, you actually don't even make a thumbprint. Simply press the Kiss into the cookie at the very end of the bake just when it comes out of the oven. You don't want the chocolate to melt.