Preheat and Prep Pan: Preheat the oven to 350°F and line a baking sheet with parchment paper.
Cream Butter and Sugar: Beat the butter and sugar until light and fluffy, about 2 minutes, scraping down the bowl as needed.
1 cup unsalted butter, 2/3 cup granulated sugar
Add Yolk and Vanilla: Mix in the egg yolk and vanilla until fully combined and smooth.
1 large egg yolk, 2 teaspoons vanilla extract
Mix in Dry Ingredients: Add the flour and salt and mix on low just until a soft dough forms. If it looks crumbly at first, keep mixing briefly—it should come together.
2 1/4 cups all-purpose flour, 1/4 teaspoon fine salt
Shape Dough Balls: Roll the dough into 1-inch balls, about 1 tablespoon of dough each, and place them 2 inches apart on the baking sheet.
Make Heart Thumbprints: Press your thumb into the center of each dough ball at a slight angle to make one side of the heart. Then press again next to it, angling inward, to create the second side. Gently pinch the bottom into a soft point to shape a clear heart. If the edges crack, lightly smooth them back together.
Quick Bake Before Filling: Bake for 7 minutes so the cookies set slightly and the wells hold their shape better.
Fill with Jam: Pull the pan out, press the centers again to deepen the wells, then fill each with about 1/2 teaspoon jam.
1/2 cup strawberry or raspberry jam
Finish Baking: Bake for 6 more minutes, until the cookies look set and the bottoms are lightly golden.
Cool: Let cookies cool on the pan for 5 minutes, then move to a cooling rack to cool completely so the jam can set.