Make this in advance for an event. It can be refrigerated for up to 5 days.
Preheat oven to 375F. Grease bottom and sides of an 8 x 8 inch pan and over bottom with parchment paper.
In a medium bowl, whisk flour, lemon zest, salt and cornstarch.
Add sugar and baking powder whisking again until evenly combined.
On a box grater, grate butter into flour mixture and mix with a fork to combine. Add egg yolk and mix until combined (mixture will be crumbly).
Press half of the mixture into prepared pan.
In a separate medium sized bowl, stir together sugar, cornstarch, lemon juice and vanilla blending until smooth. Add blueberries gently tossing to combine.
Pour blueberry mixture over crust and crumble remaining flour mixture over blueberries Bake for 40 minutes or until top is golden and sides are bubbly. Cool to room temperature before cutting into squares.