1. Preheat the Oven:

   • Preheat your oven to 180°C (350°F). Make sure the oven rack is in the middle position for even baking.
   • Grease a 9×13 inch (or similar size) baking pan with butter or non-stick cooking spray. Alternatively, you can line the pan with parchment paper for easier removal.

2. Prepare the Cake Base:

   • In a large mixing bowl, combine 250g (2 cups) all-purpose flour, 125g (2/3 cup) sugar, 1 tsp baking powder, and 1/2 tsp salt. Whisk together until well combined to ensure even distribution of the dry ingredients.
   • Add 125g (1/2 cup) softened unsalted butter. Using a hand mixer or a stand mixer with a paddle attachment, beat the butter until creamy.
   • Add 2 large eggs, 125ml (1/2 cup) milk, and 1 tsp vanilla extract. Mix on medium speed until the batter is smooth and well combined, scraping down the sides of the bowl as necessary.

3. Add Filling (if using):

   • If you’re using fresh fruit (like apples or cherries) or fruit jam, prepare it now. For fresh fruit, wash, peel (if necessary), and chop it into bite-sized pieces.
   • Pour the prepared batter into the greased baking pan, spreading it evenly with a spatula.
   • Gently spoon the fruit or jam over the cake batter, distributing it evenly. If using jam, you can swirl it slightly into the batter for a marbled effect.

4. Make the Streusel Topping:

   • In a separate bowl, mix together 150g (1 1/4 cups) all-purpose flour, 100g (1/2 cup) sugar, and 1 tsp cinnamon (if desired).
   • Pour 75g (1/3 cup) melted unsalted butter into the dry ingredients. Use a fork or your fingers to mix until crumbly, ensuring the mixture resembles coarse crumbs. The texture should be somewhat clumpy but still loose.

5. Assemble the Cake:

   • Evenly sprinkle the streusel mixture over the top of the cake batter and filling. Make sure to cover the surface well, as the streusel will form a delicious, crunchy topping while baking.

6. Bake:

   • Place the baking pan in the preheated oven. Bake for 35-40 minutes. Start checking for doneness around the 30-minute mark. A toothpick inserted into the center of the cake should come out clean or with a few moist crumbs (not wet batter).

7. Cool:

   • Once baked, remove the pan from the oven and let it cool on a wire rack for 15-20 minutes. This cooling period helps the cake set and makes it easier to slice.

8. Serve:

   • After cooling, use a sharp knife to cut the cake into squares or rectangles. Serve warm or at room temperature. Streuselkuchen pairs wonderfully with coffee or tea.

Storage

• Store any leftovers in an airtight container at room temperature for up to 3 days. For longer storage, you can freeze individual slices wrapped in plastic wrap and then in foil for up to 3 months. Thaw at room temperature before serving.