Preheat oven to 350°F.

Cream butter and sugar; beat in yolks and vanilla.

Mix in flour and salt until crumbly; bring dough together with hands.

Roll into 1-inch balls; place on a baking sheet.

Press an indent into each with your thumb or a measuring spoon's back.

Fill each with about half a teaspoon jam.

Bake 12 to 14 minutes, until edges are lightly browned.