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