Preheat oven to 390 degrees F (200 degrees C).
Prep your muffin tin with muffin liners, or by greasing the tin with oil or butter. You can bake all 12 muffins at once using a regular muffin tin, or make 6 at a time to promote a little bit more of an even rise.
Prep your apples by chopping them into small cubes - no need to peel.
Beat eggs and sugar together using a whisk for a few minutes until slightly lighter in color.
Add the milk, oil, and vanilla extract to the eggs and sugar and whisk to combine.
Combine your dry ingredients for the batter - all-purpose flour, baking powder, salt, and cinnamon - in a separate bowl.
Add the apples to the dry ingredients and mix so that the chunks of apple are coated in flour.
Add the dry ingredients to the wet ingredients and mix until no dry spots remain.
Distribute the batter evenly into the muffin tin. This recipes makes about 12 regular sized muffins, but if you would like them to be on the larger side you can use a bigger muffin pan and make 6 instead. This just may affect the final baking time! You also may want to bake 6 at a time of the regular size, but this is up to you. Alternating in the tin could help promote a more even rise.
For the streusel topping, combine all the ingredients except for the butter in a bowl - flour, brown sugar, granulated sugar, salt, and cinnamon. Mix well to evenly distribute.
Melt butter and mix with the rest of the topping ingredients, until clumps form.
Divide the topping mixture evenly over top of the muffin batter.
Bake the muffins for 5 minutes at 390 F (200 C), then lower the oven temperature to 345 F (175 C) for about 10 minutes. Check the muffins - a knife inserted to the center of one should come out clean. They may need a few minutes longer!
Let the muffins cool for a few minutes before eating and then enjoy!