Preheat a waffle iron. Whisk together hazelnut meal, oat flour, baking soda, salt, and cardamom in a medium bowl until blended. Make a well in center. Add milk, yolks, coconut oil, vanilla, and 2 tablespoons of the syrup to well, and whisk until completely blended.
Beat egg whites in a separate bowl with an electric mixer on high speed until stiff peaks form, about 2 minutes. Fold whites into hazelnut mixture. Spray iron liberally with cooking spray. Pour batter into each waffle iron cavity (use 1/3 cup for a Belgian iron; 1/4 cup for standard); cook until browned and lightly crisp on edges, about 4 minutes. Transfer to a serving plate. Repeat with rest of batter.