1. Get your oven heating up to 375°F (190°C). Place paper liners in a standard 12-cup muffin tin and put it aside for later.

Make the Streusel Topping

1. In a medium bowl, put the flour, brown sugar, cinnamon, and cold butter cubes for the streusel.
2. Use a fork or a pastry blender to mix and cut the butter into the dry ingredients. Keep going until you have a crumbly mixture that holds together when you squeeze a bit between your fingers.
3. Cover the bowl and place it in the refrigerator while you prepare the rest.

Make the Apple Filling

1. In another medium bowl, combine the diced apples, brown sugar, and cinnamon for the filling.
2. Stir everything together gently so the apples are evenly coated with the sugar and spice mixture. Set this bowl aside while you make the batter.

Make the Muffin Batter

1. In a large bowl, combine the flour, granulated sugar, baking powder, cinnamon, and salt. Whisk them together.
2. Add the cool butter cubes to the dry ingredients. Use a fork or pastry blender to cut the butter in until the mix looks like coarse crumbs.
3. Add the eggs, milk, vegetable oil, and vanilla extract to the flour and butter mixture.
4. Whisk everything together just until combined. Don’t worry if there are a few small lumps in the batter, that’s perfectly fine.

Assemble and Bake the Muffins

1. Spoon the muffin batter into the prepared muffin cups. Fill each cup a bit more than halfway.
2. Spoon some of the apple filling mixture over the batter in each cup. If the apples released juice while sitting, try to drain off as much liquid as possible before adding them.
3. Sprinkle the chilled streusel topping evenly over the apples in each muffin cup.
4. Bake the muffins in the center of your preheated oven for 15 to 20 minutes. They are ready when a wooden skewer inserted into the center of a muffin comes out without wet batter. Note the skewer might seem damp from the apples, just ensure no actual batter clings to it.
5. Let the muffins cool in the muffin tin for about 10 minutes. Afterwards, carefully move them to a wire rack to cool down completely.
6. Serve the muffins as they are, or you can add a simple drizzle of powdered sugar icing if you like.