Preheat oven to 375. Grease a standard 12-cup muffin tin, set aside.
In a medium bowl, combine flour, baking powder, cinnamon and sea salt, set aside.
In a large bowl, whisk together melted butter, vanilla, applesauce, milk and egg.
Add sugars to the wet mixture and whisk to combine.
Add dry ingredients to the wet ingredients and stir until combined.
Add diced apples and stir until evenly distributed.
Make crumb topping:
Combine butter, sea salt, cinnamon, brown sugar and all-purpose flour with a fork or pastry cutter.
Add oats and use a fork to combine until evenly distributed.
Putting it together
Use a 2 TBS cookie scoop to measure portions of muffin batter into each well of the prepared muffin tin.
Add about 1 TBS of streusel topping to each muffin, spread it over the top of the batter and gently press it into the batter.
Bake at 375 for 18-22 minutes or until the tops spring back when touched and a cake tester inserted in the center comes out with a few moist crumbs
Once muffins are baked, remove them from the oven and use a spoon or knife to immediately remove any streusel that baked onto the pan. Push in the sides of each muffin gently to release them from the muffin pan.
Let the muffins cool in the muffin tin on a wire rack for 10 minutes, then to a cooling rack.