Combine dry ingredients: In a large mixing bowl, whisk together the flour, baking powder, salt, and sugar.

Prepare wet ingredients: In another bowl, whisk together the milk, eggs, melted butter or oil, and vanilla extract.

Combine wet and dry: Pour the wet ingredients into the dry ingredients and mix until just combined. Be careful not to overmix; some lumps are okay. Gently fold in the blueberries.

Cook waffles: Lightly grease the waffle iron if needed. Pour an appropriate amount of batter onto the preheated waffle iron and cook according to your waffle maker's instructions until golden brown. Repeat with the remaining batter.

Cook blueberries: In a saucepan over medium heat, combine the blueberries, water, honey or maple syrup, lemon zest, and lemon juice. Stir gently to combine.

Simmer: Bring the mixture to a simmer, stirring occasionally. Let it cook for about 8-10 minutes, or until the blueberries have burst and the mixture has thickened slightly.

Add vanilla: Remove the saucepan from heat and stir in the vanilla extract. Adjust sweetness or tartness by adding more sweetener or lemon juice if desired.