Preheat a waffle iron to medium-high heat. Grease with nonstick spray.
OPTION 1: If using prepared mix, in a medium bowl, whisk together the waffle mix, milk, egg, and vegetable oil until fully incorporated.
OPTION 2: If making the waffle batter from scratch, in a medium bowl, sift together the flour, sugar, baking powder, baking soda, and salt. In a small bowl, whisk together the eggs, melted butter, and buttermilk. Whisk the wet ingredients into the dry ingredients until fully combined.
Add 1 cup (130 g) of waffle batter to the center of the waffle iron. Cook until the waffle is golden brown and cooked through, 4–6 minutes. Repeat with the remaining batter.
Top each waffle with 1 tablespoon maple syrup and ½ teaspoon powdered sugar. Garnish with ¼ cup (45 g) mixed berries and 2 tablespoons walnuts.