Preheat the oven to 350 degrees and grease a small 8x8 baking dish.
In a mixing bowl, stir together 1 cup flour, granulated sugar, baking powder, and salt.
Add the softened butter, egg, and milk to the dry mix and stir until just combined into a smooth batter.
Fold in the chopped apple and 1 teaspoon cinnamon until evenly distributed.
Pour the batter into the prepared dish and spread it level.
In a separate bowl, mix the remaining 1/4 cup flour, brown sugar, and 1/2 teaspoon cinnamon.
Cut in the cold butter cubes until the mixture forms coarse crumbs, then sprinkle evenly over the batter.
Bake for 30 to 35 minutes until a toothpick inserted in the center comes out clean and the streusel looks golden.