Poppy Seed Cake with Caramel Frosting

Name: Poppy Seed Cake with Caramel Frosting
Cuisine: American
Ingredients: Butter and flour, for the cake pans; 1 cup poppy seeds; 1 cup sour cream; 2 cups all-purpose flour; 2 teaspoons baking powder; ¼ teaspoon ground cardamom; 1 teaspoon grated lemon zest; 4 egg whites; 2 cups brown sugar; 1 cup granulated sugar; 1½ cups heavy cream; 3 tablespoons butter