"Streusel" is a crumbly topping made from a mixture of butter, flour, golden sugar, and ground cinnamon.  "Streusel" was originally used as a topping for the "StreuselKuchen" but you can use "Streusel" for any coffee cakes, muffins, tarts, dimsum bread, etc. etc.
Ingredients:

500 grams Fresh gooseberry
300 grams Fresh blueberry
2 tablespoons Freshly squeezed lemon juice
150 grams Golden caster sugar
350 grams German #405 flour
¼ teaspoon Salt
1 teaspoon Cinnamon, ground
1 teaspoon Baking powder
200 grams Cold Kellygold butter
1 Egg, lightly beaten
½ teaspoon Vanilla extract

Procedure:

Step 1 - Line a 30cmx25cm rectangle baking tray with parchment paper. Preheat the oven to 190C/375F.

Step 2 - Combine the gooseberry, blueberry, lemon juice, half of the sugar and 2 tablespoon of flour in a mixing bowl.

Step 3 - In another bowl, mix together the rest half of the sugar, flour, salt, ground cinnamon and baking powder. Cut in cold butter and rub until coarse crumbs form. Add in the egg and vanilla extract.

Step 4 - Place half of the mixture into the prepared baking tray. Spread on the berry mixture and cover with another half of the flour mixture. Bake for about 40-45 minutes until the surface has become golden and crisp.