whisk together flour, salt, and cardamom in a bowl. Set dry ingredients aside.
Whisk together eggs, milk, and orange and lemon zest in a measuring cup or bowl with a pouring spout.
Add flour mixture to egg mixture. Whisk to mix. Melt 1 tablespoon butter, whisk into popover batter until batter is smooth. Cover batter. Let rest at room temperature for 30 minutes.
Preheat oven with a 6 cup popover pan inside, to 450°.
Remove hot popover pan from oven. Brush cups with the remaining 1 tablespoon butter. Immediately pour batter into popover cups, filling two thirds full. Return pan to oven for 15 minutes. Reduce heat to 375°. Bake popovers another 20 minutes.
Turn off oven. Pierce popovers with a paring knife to release steam. Return popovers to the oven. Close oven door. Leave popovers in the closed oven for 10 minutes more to dry in sides. Serve at once.