Preheat your waffle iron according to the manufacturer's instructions (usually to medium-high). Lightly spray the plates with nonstick spray or brush with a thin layer of oil if needed.
In a large bowl, whisk together 2 cups all-purpose flour, 2 tablespoons packed brown sugar, 2 teaspoons baking powder, 1/2 teaspoon ground cinnamon, and 1/2 teaspoon salt until evenly combined.
In a separate bowl, whisk 1 3/4 cups plain nonfat Greek yogurt and 1 cup milk until smooth and slightly loosened. Whisk in 2 large eggs until blended, then whisk in 4 tablespoons salted butter (melted and cooled) and 1 teaspoon vanilla extract.
Pour the wet ingredients into the dry ingredients. Stir gently with a spatula or whisk until just combined; small lumps are okay—do not overmix.
Spoon or pour batter onto the preheated waffle iron (amount per waffle will depend on your iron). Close the lid and cook until the waffles are golden and cooked through, about 3 to 5 minutes, or according to your waffle iron's instructions.
Transfer cooked waffles to a plate or wire rack to keep crisp. Serve immediately topped with butter, maple syrup, and fresh fruit as desired.