Step 1

Beat butter in a bowl with an electric mixer until light and fluffy. Add egg yolks to butter one at a time, blending well after each addition.

Step 2

Mix flour, white sugar, baking powder, cardamom, and salt together in a bowl. Stir flour mixture into butter mixture until just combined. Divide dough in half and wrap each tightly in plastic wrap. Freeze for at least 3 hours.

Step 3

Preheat oven to 350 degrees F (175 degrees C).

Step 4

Grate 1 ball of frozen dough evenly into the bottom of a 9x13-inch baking pan. Spread blackberry jam over grated dough, leaving 1/2-inch space around the edges. Grate the second ball of frozen dough over the jam.

Step 5

Bake in the preheated oven until golden brown, 30 to 40 minutes. After shortbread cools, sprinkle with confectioners' sugar and cut into bars.