Preheat oven to 190°C (375°F). If you want the same size of the bars as you can see in the picture then place parchment paper in a baking dish with dimensions: 18 cm (7 inches)*24 cm (9,5 inches)
Take a medium bowl and combine 50 g of sugar with the cornstarch. Stir in the lemon juice and then add the blueberries. Gently toss to coat the blueberries evenly then set it aside
Take a large bowl and mix all dry ingredients (except the muscovado sugar)
Add the lemon zest, the egg yolk and the butter then use your fingers to work the mixture until you have evenly sized crumbs
Spread 2/3 of the batter into the baking dish, then add the blueberry mixture on top.
Mix the rest of the batter with the muscovado sugar and add it as a top layer (it should be enough to coat everything)
Place in oven and bake for around 40 min (crumbs should then be browned but not burnt)
Let it cool down before cutting it to bars. You can now store it in the fridge for a couple of days. Serve as is or with a scoop of vanilla ice cream