Preheat the oven to 180 degrees C.
Line an 8x8 inch square baking pan with aluminium foil leaving an overhang on each side and grease with butter.
To prepare the blueberry filling, combine the sugar, water, blueberries, and lemon zest in a saucepan.
Bring to a boil over medium heat. Simmer for 5-10 minutes or until the sugar is melted and blueberries are starting to burst. Set aside.
In a separate large bowl whisk together the flour, granulated sugar and baking powder.
Add the cold butter and the egg yolk. Mix using your fingers until the mixture is crumbly and
resembles wet sand.
Press about two thirds to three quarters of the mixture into the bottom of the prepared pan.
Press it gently with the pressure of your hand. Pour the blueberry mixture over top.
To the remaining flour mixture, add the brown sugar and mix.
Crumble the remaining dough mixture using your fingers.
Sprinkle it over the blueberry filling layer. It should nicely cover the blueberry layer.
Bake in the oven for about 45 minutes, until you see the blueberries bubbling and the crumble
layer starting to turn golden.
Allow it to cool down to room temperature. Cut the crumble bars into squares.
You can enjoy them immediately or store at room temperature in an airtight box.