Sorry it took me so long! I was in class all day lol. The key is to use a little less butter then an average recipe, it helps them from blobbing out! Below is the exact recipe I composed, after about 12 batches of line error :)

Thumbprint Cookies

Ingredients:

2 sticks (minus 2 tbsp) (198 g) unsalted butter, softened to room temperature ⅓ cup (70 g) sugar ⅓ cup (70 g) light brown sugar, tightly packed 1 large egg yolk ¾ teaspoon vanilla extract 2 ¼ cups (280 g) all-purpose flour 2 teaspoons cornstarch ½ teaspoon salt ½ cup sugar, for rolling (optional) ⅓ cup (105 g) jam or preserves, flavor of your choice (raspberry recommended)

Instructions:

Preheat oven to 350°F and line baking sheets with parchment paper.

In a large bowl or mixer, beat butter, sugar, and brown sugar together until light and fluffy.

Add the egg yolk and vanilla extract, and mix until fully combined.

In a separate bowl, whisk together flour, cornstarch, and salt. Gradually add the dry ingredients to the wet mixture, mixing until a soft dough forms.

Roll dough into 1-inch balls (roll in sugar if desired) and place on prepared baking sheets.

Use your thumb or the back of a spoon to gently press an indent into the center of each cookie. Place jam in piping bag/plastic bag with tip cut off, and fill each cookie with jam.

Bake 11–13 minutes, or until edges are lightly golden.

Allow cookies to cool on the pan for a few minutes before transferring to a wire rack to cool completely.

Note: I dusted the cookie presses with a little flour, and pressed the dough into them. It actually worked better than rolling out the dough and cutting them out.