Preheat oven to 350°F. Grease a 9-inch square tin or baking dish and line with parchment paper. You can also grease the parchment to further prevent sticking. Place two oven racks on the middle two positions.

Make the crumble. In a medium mixing bowl, whisk together the flour, cocoa powder, sugar, brown sugar, baking powder, and salt, breaking up any lumps you see. Fold in the melted butter with a rubber spatula until there are no dry bits and the mixture is forms pea-sized crumbles. If the mixture is too dry, add about a tablespoon of water. (To get bigger crumbles, just squeeze the dough together in your fist and lightly break it apart.)

Dump about one half to two thirds of the crumble mixture into the bottom of the tin. Using an offset spatula or the bottom of a measuring cup, press the dough into the tin, completely covering the bottom in an even layer. Spread the rest of the crumbles evenly onto a parchment lined baking sheet.

Bake both trays for 5 minutes, then remove from the oven and turn the heat down to 300°F. Brush the still-hot bottom crust with the reserved egg white.

For the filling, in a small bowl, whisk the eggs, egg yolk and salt until very smooth. In a medium mixing bowl, combine the sugar and the zests, rubbing the zest into the sugar with your fingers. Whisk in the flour until there are no lumps, then the egg mixture, then the blood orange and lemon juices, until homogenous.

Strain the filling onto the baked bottom crust and bake for 20-25 minutes, just until the top of the filling is set but still wobbles slightly in the middle. Gently and evenly scatter the crumbles on top and bake for another 5 minutes.

Transfer to a wire rack to cool completely, cover, then transfer to the fridge to chill before serving.