Heat oven to 350°F; line two baking sheets with parchment.
Cream butter and sugar in a large bowl about 3 minutes until light and fluffy. Beat in yolks and vanilla.
Whisk flour and salt separately; gradually mix into butter just until combined.
Roll into 1-inch balls; space about 2 inches apart on sheets.
Indent centers with thumb or small measuring spoon. Fill each with about 1/2 teaspoon jam.
Bake 12-14 minutes until edges are lightly golden; cookies remain soft but firm while cooling.
Cool on sheet 5 minutes; transfer to rack to cool completely. Dust with optional powdered sugar before serving.