Preheat the oven to 350°F (convection) or 375°F (conventional) and line a baking sheet with baking paper.
Start by preparing the cherry filling for the puff pastry streusel squares. Drain the cherries and save the juice for later. In a saucepan, bring the juice and 1 tablespoon of sugar to a boil.
In a small bowl, whisk the cornstarch with 1-2 tablespoons of water until smooth. Remove the saucepan from the heat as soon as the juice boils. Stir in the cornstarch until smooth. Add the cherries and bring to a boil again, stirring constantly.
The second step is to prepare the streusel. Combine the softened butter, flour, and remaining sugar. Knead the mixture with your hands or a hand mixer until streusel forms, it should be crumbly. The longer you knead, the smaller and finer the streusel will be.
Now it's time to make the streusel squares. Roll out the puff pastry and cut it into equal-sized squares. Spread the cherry filling generously over the puff pastry and sprinkle it with the streusel.
Place the streusel squares on a baking sheet and bake for 25-30 minutes.
