Preheat your oven to 375°F and grease a 9×13-inch baking dish with 4 tablespoons of salted butter.
In a large bowl, whisk together the granulated sugar, light brown sugar, and lemon zest.
Add the flour, cinnamon, and salt, mixing well.
Stir in the vanilla syrup, lemon juice, and vanilla extract until thick.
Toss in the blueberries until coated, then pour into the baking dish and bake for 15 minutes.
While the blueberries bake, prepare the topping by pulsing the flour, sugar, and baking powder in a food processor.
Add the cold butter and pulse until crumbly.
Mix in the buttermilk and vanilla until just moistened.
Drop clumps of the topping onto the hot blueberries and bake for an additional 32 to 37 minutes until golden brown.
Serve warm or let cool for a thicker texture.