Preheat oven to 375 degrees F. Line a cookie sheet with parchment paper.
Beat together butter and sugar on medium-high speed until light and fluffy, about 2 to 3 minutes. Beat in egg yolks and vanilla extract.
Add flour and salt and mix until incorporated and dough comes together in a ball. If desired, chill for 30 minutes.
Form dough into 1-inch balls (make sure they’re perfectly round), and arrange on prepared cookie sheet. Use a 1/2 tsp. measure to create perfectly circular craters in the center.
Bake cookies for 8 to 10 minutes.
Remove cookie sheet from oven. If indentations look shallow, further define them again, and fill each with a bit of jam.
Bake for an additional 3 to 4 minutes, or until jam melts slightly and edges of cookies are lightly golden.