Step 1

Preheat the oven to 300ºF. Prepare a 10-x-15-inch jelly roll pan by greasing lightly with cooking spray or butter.

Step 2

Whip the egg whites with an electric mixer until they form stiff peaks. Gently transfer to a separate bowl and set aside.

Step 3

Use the mixer to cream the butter and sugar. Add vanilla and egg yolks, and beat for a full three or four minutes, until their volume has increased and the mixture is yellow and smooth.

Step 4

Add the flour, baking soda, baking powder, and salt. Stir into the butter mixture with a wooden spoon or your hands. Pat it out in the prepared pan, crimping it into the corners; if the dough is a little crumbly just press it in firmly.

Step 5

Spread the dough with jam; sprinkle 1/2 cup ground walnuts over the jelly and spread beaten egg whites evenly over the nuts. Sprinkle the remaining nuts over egg whites.

Step 6

Bake for 30 minutes. Watch carefully and avoid over-baking the egg white topping. It should be crisp but not really brown at all. The crust should be just barely golden brown.

Step 7

Cut into squares when cool. Store in a closed container at room temperature for up to 5 days. These are best on the day they are made, as the egg white topping softens after a day.