Step 1Heat the oven to 350 degrees and grease a 9-inch-square baking pan. Line the bottom of the pan with a piece of parchment paper and grease again.
Step 2In the bowl of a stand mixer fitted with the paddle attachment, or in a large bowl using a hand mixer, beat the butter and sugar until light and fluffy, 3 to 5 minutes. Beat in the egg yolks until combined. Beat in the flour, then the salt. Stir in the chopped walnuts by hand.
Step 3With lightly floured hands, divide the dough in half. Pat one half evenly onto the bottom of the prepared pan. Spread the jam over the dough. Pinch off small pieces, about three-fourths inch in diameter, of the remaining dough and drop it over the jam; do not pat the dough down.
Step 4Bake the dough until golden, 45 to 50 minutes, rotating halfway for even coloring. Cool completely in the pan on a wire rack, then cut lengthwise into three strips. Cut each strip crosswise into 10 pieces.