Preheat oven to 375 degrees F (190 degrees C). Grease one 11x13-inch rectangular baking dish.
Whisk flour, baking powder, cardamom, and salt together in a bowl.
Whisk eggs together in a small bowl. Discard 1/4 of the egg mixture. Combine remaining egg mixture, butter, and sugar in a large bowl; beat with an electric mixer on medium speed until smooth, about 3 minutes. Fold the flour mixture into the egg mixture slowly. Beat briefly on low speed just until dough comes together.
Transfer dough to the baking dish. Dust your hands with flour and pat dough into an even layer about 3/4-inch thick. Prick dough in several places with a fork and sprinkle the nigella seeds evenly on top.
Bake in the preheated oven until lightly golden, 20 to 25 minutes. Remove from the oven and run a knife around the edges to loosen it from the pan; cut roht into squares while still warm. Let cool.