In a small bowl using a hand mixer or in the bowl of a stand mixer with the whisk attachment, beat egg whites and 2 tablespoons of sugar until light and fluffy and form soft peaks. Set aside.
In a large bowl, whisk flour, the remaining 2 tablespoons of sugar, baking powder, baking soda, and salt. Make a well in the center of the batter and pour in melted butter and buttermilk. Whisk until mixture is combined and no lumps remain.
Add 1/4 cup of the blackberry lemon jam to the waffle batter. Stir to mix throughout.
Gently fold in the egg whites with a spatula. Be careful to mix until just incorporated. Do not overmix.
Using a 3/4 cup measuring cup (see notes), scoop batter and pour onto heated waffle iron. Cook until waffle is browned and crispy on the outside, about 5 minutes, or when the timer goes off on the waffle iron.
Repeat for the remaining waffles. As the waffles are done, place inside microwave or in a warming drawer to keep warm until you serve.
Once waffles are done cooking, serve with additional blackberry lemon jam on top or with maple syrup.
Notes
The amount of waffle batter you measure will depend on what type of waffle maker you’re using. I use a Belgian waffle maker. To make sure you don’t overflow the waffle maker, start with a smaller amount of batter and then build up.