1 c. chilled unsalted butter, cut into 1/2″ pieces
1/2 c. granulated sugar
1/4 c. powdered sugar
1/2 tsp. kosher salt
1/2 tsp. ground cardamom
2 1/2 c. flour
2 tsp. finely grated orange zest
1 egg, beaten to blend
Coarse sugar for sprinkling
Preheat oven to 350 degrees.
Using an electric mixer on medium-high speed, beat butter, granulated sugar, powdered sugar and salt until very light and fluffy, 7 – 10 minutes. Reduce the speed to low and add flour, cardamom and orange zest. Mix just to combine. Dough will look a little dry.
Press the dough into 2 8″ square cake pans. Brush with egg and sprinkle with coarse sugar.
Bake until the shortbread is golden brown and the sides pull away from the pan, about 20 – 25 minutes. Transfer pan to a wire rack; let cool in pan before turning out and cutting into gars.
Shortbread dough can be made 1 month ahead; wrap tightly in plastic wrap and freeze.