Preheat your oven 375 degrees and line a large baking sheet with parchment or a baking mat. Set aside.
In a small bowl add the egg yolk, heavy cream, and vanilla. Beat with a fork until combined.
In the bowl of a stand mixer add the butter and granulated sugar. Beat on medium speed until the mixture is light and fluffy, about 3 minutes. Add the yolk mixture and beat to combine, about 30 seconds. Scrape down the sides of the bowl as needed. Add the salt and flour and mix until the dough comes together.
Using a tablespoon scoop, scoop out a ball of dough and place it on the sheet. Place the cookies about 1 ½ inches apart. Use your thumb or a rounded measuring spoon to make an indent in the center of the cookies. Press gently with your thumb until the indent in the cookie ball is about an inch across, repeat this step with each cookie dough ball.
Add ¼ to ½ teaspoon of jam to each indent. Put the prepared cookie sheets in the fridge and let the cookies chill for 15 to 20 minutes.
Bake the cookies for 10-12 minutes until the bottom edges start to brown. Remove from the oven and allow the cookies to cool on the pan for 10 minutes. Remove and let them finish cooling on a wire rack. Store extra cookies in an airtight container for up to 5 days.
Notes
You can use just about any jam or jelly you like. I used homemade raspberry jam, peach jelly, and strawberry freezer jam. They all worked great and they all tasted great too. Use your favorite and feel free to use a variety.
The key to the crisp and tender cookie is chilling the dough before baking it. This butter-heavy dough will spread really bad and the cookies will look ugly if you don't chill them before baking.
Don't limit yourself to one jam or jelly! Feel free to use a variety or even create a rainbow of flavors on one baking sheet for a colorful treat.
If you don't want to use your thumb for the indent, the back of a rounded measuring spoon works great. Just ensure the indent is deep enough to hold the jam but not so deep that you pierce through the cookie.
While these cookies taste best fresh, they can be stored in an airtight container for up to 5 days. If you notice them softening, you can re-crisp them in a low oven for a few minutes.
It cannot be stressed enough how essential it is to chill the dough. This step ensures a crisp, tender cookie. Skip this step, and you risk having cookies that spread too much and lose their shape.