Pre-heat the Oven to 350°F (176°C).
Prepare the Streusel: In a medium bowl, combine the flour, sugars, cinnamon, and softened butter. With your fingers, or a fork, mix all the ingredients together until crumbly. Put the mixture in the refrigerator while preparing the cake.
1 cup all-purpose flour, 2 tbsp brown sugar, 6 tbsp softened butter, ½ tsp ground cinnamon, ½ cup granulated sugar
Mix Dry Ingredients: Set aside 1 tbsp of flour from the total amount. In a large bowl, whisk together the flour, cornstarch, baking powder, baking soda, and salt.
2 cups all-purpose flour (set aside 1 tbsp used for dusting), 1 tbsp cornstarch, ½ tsp baking soda (bicarbonate of soda), 1 tsp baking powder, a pinch of salt
Make the batter: In a separate bowl, whisk the sugar with the melted butter (make sure the butter has cooled off before using it). Add in the eggs, vanilla extract, and sourdough discard. Mix until well combined.
2 large eggs (room temperature), ⅔ cups melted butter (unsalted), ⅔ cups granulated sugar (I used unrefined), 1 tbsp vanilla extract, ½ cup sourdough discard
Combine Wet and Dry: Gradually incorporate the flour into the batter and stir just until combined. Be careful not to overmix! Use a spatula for this.
Prepare and fold in Strawberries: Dice strawberries into small chunks. Mix them with that tablespoon of flour you set aside.
2 ½ cups fresh strawberries (roughly chopped)
Assemble and Bake: Pour the batter into a 8-inch (or 9-inch) square baking dish lined with baking paper. Layer the strawberries on top. Sprinkle the streusel topping evenly over the strawberries.
Bake in a preheated oven at 350°F (176°C) for about 1 hour, or until a toothpick inserted into the center comes out clean.
Cool and Enjoy: Let the coffee cake cool in the pan for a few minutes before transferring it to a wire rack to cool completely. Slice and serve warm or at room temperature.