Preheat the oven to 200 degrees C. Butter and base line a 20 cm square tin or a 25 cm X 15 cm rectangular tin with baking paper, with a slight overhang over 2 opposite ends.
For the base, put the flour, baking powder and sugar in a food processor and blitz. Cut the cold butter into small cubes, add to the processor and blitz again, then add the egg and pluse until it forms a crumbly dough. Alternatively, you can make by hand using the rubbing in method. Tip the crumbly dough into the tin and press well into the base with the back of a spoon. Bake for 15 minutes.
Meanwhile, make the filling and topping.
To brown the butter, put the butter in a saucepan over a low heat. Once it has melted, increase the heat to medium and cook for about 5-8 minutes, stirring often, until it bubbles and turns foamy and then golden brown. It should also smell nutty. When it reaches this stage, remove the pan from the heat and strain through a sieve into a bowl.
Wash the blueberries and remove any stalks, then put them in a bowl. Add the lemon / lime juice, finely grated lemon / lime zest and sugar, then sift over the cornflour. Leave for a few minutes while you make the crumble topping.
For the crumble topping, add the sugar and ginger / cinnamon to the browned butter, stir well, then sift over the flour and stir again to form a soft damp crumble.
Pour the blueberries, and any juice, on top of the par-baked base, then scatter the crumble over the top.
Bake for 30 minutes, or until the top is crisp and the fruit bubbling underneath. Leave until it is completely cold (about 3 hours) before removing from the dish / tin and cutting into squares.