Place egg yolks into a large mixing bowl with the sugar, and beat until light and frothy. Mix in the vanilla sugar and water until evenly combined, and stir in half the melted butter, half the buttermilk, half the flour. When mixture is smooth, mix in remaining butter, buttermilk, flour, cardamom, and salt, and beat again to a smooth batter.
In a separate bowl, beat the egg whites with an electric mixer until they form stiff peaks; gently fold the egg whites into the batter, incorporating as much volume as possible.
Preheat a waffle iron according to manufacturer's directions.
Spoon about 1/3 cup of batter into the preheated waffle iron, close the lid, and cook until the waffle is golden brown, about 5 minutes. Remove waffle to a plate. If using a heart-shaped iron, break the waffle into individual hearts to serve. Repeat with remaining batter.
