Preheat the oven to 375F degrees. Line an 8x8 inch square baking pan with aluminium foil and grease with butter or shortening.

In a medium bowl toss together the ingredients for the blueberry layer (blueberries, cornstarch, lemon zest, lemon juice & sugar). Put aside.

In a separate large bowl whisk together the flour, 1/2 cup granulated sugar and the baking powder.

Cut in the butter & egg yolk using a fork until the mixture is crumbly.

Press about two thirds to three quarters of the mixture into the bottom of the prepared pan. Pour the blueberry mixture over top.

To the rest of the flour mixture, stir in the brown sugar and remaining 1 tablespoon of granulated sugar.

Crumble the remaining dough over the blueberry mixture using your hands to distribute on the top.

Bake in the oven for about 45 minutes, until you see the blueberries bubbling and the crumble layer starting to turn golden.

Allow to cool fully to room temperature and ideally put in the refrigerator to chill . Then lift the bars out of the pan using the aluminium foil overhang and cut into squares. I used a pizza cutter which worked well. I also let the bars warm to room temperature which really brings out the flavors.

Enjoy!