Preheat oven to 350°F. Line a half pan baking sheet (18"x13" with 1" lip) with parchment paper, making sure it extends up to the sides and creased at the corners. Rubbing the bottom of the sheet with a little bit of butter will help keep the parchment paper in place when pouring the batter.
Combine flour, granulated sugar, unsalted butter and vanilla sugar in a food processor or in a medium bowl. Pulse/Process or crumble with your fingers until the entire mixture is the texture of coarse sand. It helps to add the butter in 3 batches.
Separate and save one cup of the crumb to use as a topping.
Mix baking powder to the remaining crumb mixture. Beat eggs and milk together in a small bowl then add to the mixture. Stir or pulse in the food processor until well combined; a few small lumps are to be expected.
Pour the batter into the prepared baking sheet and level. Carefully place and distribute the blueberries on the batter, then sprinkle the cup of crumb mixture evenly on top.
Place in the oven and bake for 40-45 minutes. It is done when the edges start to turn golden brown.
Let it cool in the pan. Can be served warm or at room temperature.**