Stir together yeast and half of the milk in a small bowl. Add 2 tablespoons flour and 1 teaspoon sugar and place the yeast mixture in a warm place for 1 hour until foamy.
Preheat oven to 200 C / Gas 6. Grease a baking tray or line with baking parchment.
Combine 500g flour, 50g sugar, butter, egg and salt and mix together in a large bowl. Add yeast mixture and knead until smooth and elastic. Place dough into a lightly greased bowl, cover with a damp cloth and let rise for about 1 hour in a warm place, until doubled in size.
Roll out the dough to fit the baking tray and line it. Top the dough with blueberries and lightly press them into the dough. Cover with a cloth and let rise for another 30 minutes in a warm place.
For the streusel:
Mix the flour and sugar in a bowl and rub in the cold butter until it resembles breadcrumbs. Sprinkle the streusel on top of the blueberries.
Bake the cake for about 30 minutes in the preheated oven until golden brown.