Preheat oven to 350. Line muffin tin with liners (I suggest the ones with the high sides, to keep the crumbs contained.); set aside.
For the muffin base, combine all the dry ingredients, stir to combine, set aside.
Combine wet ingredients and whisk until well combined.
Add wet ingredients to dry ingredients, and mix until well combined. Set aside.
To make crumb topping, combine all dry ingredients in a mixing bowl and set aside.
Melt butter; add butter to dry ingredients.
Using your hands, mix the butter and dry ingredients together until they start coming together. Squeeze the mixture into desired size chunks (I start with large chunks of crumb and then break them apart).
To the lined muffin tin, add muffin mixture to just under the top of the muffin cavity (mine takes about 2 heaping spoon fulls, about 1/4c of the mixture).
Add a generous portion of crumbs (scant 1/4c of the crumb mixture) to the filled muffin liners. You want the muffin to be well covered in crumb so that as the muffin expands the crumb still covers most of the top.
Bake the muffins for 30 minutes or until a toothpick comes out clean. Remove liners and continue with remaining batter and crumb topping.
Allow to cool completely before serving.
Keep in an air tight container for 4-6 days.