For the syrup: Combine 1 Tbsp of water and cornstarch in a small dish and set aside. In a medium saucepan bring water, sugar, and blueberries to a boil. Stir occasionally. Reduce heat and simmer for about 10 minutes. Add the water+ cornstarch mixture and mix to combine. Turn off heat and set aside to cool.
For the crumble: Stir together ½ cup flour, ½ cup sugar, and a pinch of salt. Add ¼ cup unsalted butter diced into cubes and work the butter into the flour mixture with your fingers until a crumbly mixture has formed. Set aside.
For the muffins: Preheat oven to 375°F. Add the sugar and lemon zest to a large bowl. Add the butter and mix until light in color. Add the eggs, one at a time, beating well after each addition. Then add vanilla and mix. In a small bowl sift together the flour, salt, and baking powder. Add the dry ingredients to the creamed mixture alternately with the milk. Fold in the blueberries. Line a 12 cup muffin tin with cupcake liners, and fill with batter. Add the blueberry syrup on top of the muffins and sprinkle the crumble before baking. Bake at 375°F for about 30-35 minutes.
