In a large bowl, stir together the flour, sugars, salt and cinnamon.
Melt the butter and allow to cool just slightly so it's still melted but not hot.
Stir the butter into the dry ingredients and do not break up clumps unless they are huge. Set aside.
For the Muffins
Heat the oven to 350 degrees and line 3 muffin tins with paper liners. These work best when muffins are involved, see post. See note on muffin tips for why we use three pans.
In a standing mixer, beat the butter and sugar until fluffy, about 1-2 minutes. Add the oil and mix again.
Crack in your eggs and add the vanilla extract and then give the mixer a final turn before dumping in all dry ingredients. Mix until almost incorporated and then add in the buttermilk and stir until barely combined.
Make a well in the center of the batter, add about 1 cup of the streusel and 1 cups of apples and cover it back up with batter.
Sprinkle about 1 1/2 cups more streusel on top along with the remaining apple, and using a rubber spatula, gently fold it all in.
Using a cookie scoop, add the batter evenly into eachpaper liner and sprinkle generously with streusel.
Bake 15-20 minutes, or until a toothpick in the center comes out clean.
Allow to cool slightly then drizzle with the icing and serve warm. (see note)
In a bowl, whisk together the powdered sugar, vanilla and heavy cream until a steady ribbon of drizzle comes off the whisk. Set aside.