Preheat the oven to 400°F. Spray a 9x13-inch pyrex pan with nonstick spray.
PREPARE THE CRUST:
In a medium bowl, stir together the flour, sugar, zest and salt. Cut in the butter until the mixture resembles coarse crumbs. In a separate bowl, beat the egg and vanilla together; stir into the crumb mixture until a dough forms. Press into the bottom of the prepared pan. Bake 12 to 15 minutes until golden. Remove from the oven and set aside to cool slightly. (Keep the oven on)
ADD THE FILLING:
Sprinkle the blueberries over the crust. In a small bowl, combine the 1/4 cup sugar and nutmeg; sprinkle over the blueberries.
PREPARE THE TOPPING:
In a medium bowl, use a rubber spatula to cream together the 5 tablespoons butter and brown sugar until smooth. Mix in the flour, so that the mixture is crumbly like streusel. Sprinkle over the blueberry layer.
Bake for 20 to 25 minutes until browned. Cool and then dust with powdered sugar before cutting into bars.