01 - Combine blueberries, water, sugar, and lemon juice in a medium saucepan. Bring to a simmer over medium heat, stirring occasionally. Simmer for 8-10 minutes, gently mashing some berries to release their juices. For thicker consistency, stir in the cornstarch mixture and cook 1-2 minutes until thickened. Remove from heat and set aside to cool slightly.
02 - Preheat your waffle iron according to the manufacturer's instructions.
03 - In a large mixing bowl, whisk together flour, poppy seeds, sugar, baking powder, baking soda, and salt until well blended.
04 - In a separate bowl, whisk eggs, buttermilk, melted butter, lemon zest, lemon juice, and vanilla extract until thoroughly combined.
05 - Pour the wet ingredients into the dry ingredients. Gently stir with a spatula or wooden spoon until just combined—some small lumps are acceptable. Do not overmix as this can result in tough waffles.
06 - Lightly grease the preheated waffle iron with oil or nonstick cooking spray. Pour appropriate amount of batter onto the iron and cook according to manufacturer's instructions until waffles are golden brown and crisp, approximately 4-5 minutes per batch.
07 - Serve waffles immediately while warm, topped generously with the prepared blueberry syrup.