Stir flour, sugar, baking powder, cinnamon, and salt together in a large bowl. Mix in egg, oil, and milk. Fold in chopped apples.

Spoon ½ cup of batter into each muffin liner.

To make the streusel topping: In a small bowl, mix sugar, flour, butter, and cinnamon with a fork until clinging together. Sprinkle over the muffins.

Bake in the preheated oven for 5 minutes. Then reduce oven temperature to 350 degrees F (175 degrees C) and bake until a toothpick comes out clean, about 25 to 30 minutes. Let cool in the pan briefly, then remove muffins to cool on a wire rack.