To prepare a German plum cake with streusel recipe, first you need to gather your ingredients. First, you'll need the following ingredients: 1 cup (240 ml) sugar, 1 tablespoon (25 g) flour, and 1 teaspoon (2.5 oz) vanilla extract. Mix the dry ingredients together with the wet ingredients until the mixture is smooth. Pour the batter into the prepared cake pan. Bake the cake for 35 to 40 minutes, or until it's just set. Remove from the oven and let cool for 10 minutes before cutting into squares.