Preheat the waffle iron according to the manufacturer's directions.
In a large bowl, whisk together 1 cup all-purpose flour, 1 cup whole wheat flour, 1 teaspoon baking powder, 1/2 teaspoon baking soda, 1 teaspoon salt, and 3 tablespoons light brown sugar. If the brown sugar has clumps, use clean hands or a fork to break them up, then whisk again.
In a medium bowl, beat 3 large eggs until combined. Stir in 1/4 cup melted unsalted butter (slightly cooled), then whisk in 2 cups buttermilk and 1 teaspoon pure vanilla extract until the wet mixture is uniform.
Pour the wet ingredients into the dry ingredients and gently stir with a spatula or whisk until just combined. Small lumps are fine—do not overmix.
Lightly spray or grease the preheated waffle iron following the manufacturer's guidance. Pour the recommended amount of batter for your waffle iron onto the center of the grid (enough to cover without overflowing). Close the iron and cook until the waffle is golden and releases easily from the iron.
Remove the waffle and serve immediately, or keep cooked waffles warm in a 200°F oven until ready to serve.
Serve with berries, whipped cream, and maple syrup, if desired.