Preheat oven to 375 degrees. Line muffin pan(s) with paper cups. Set aside.
In the bowl of a stand mixer, combine the flour, sugar, baking soda, and salt. Using the paddle attachment, cut in the butter, one Tablespoon at a time, until the mixture resembles fine crumbs.
In a separate bowl, whisk together the egg, egg yolk, vanilla, buttermilk, and oil. Add the wet ingredients to the dry ingredients and mix until incorporated.
Fill muffin cups ½ full. Do not overfill! About 13 to 14 muffins depending on the size of the cups. Set aside.
Prepare streusel by combining the flour, sugar, cinnamon, salt, and melted butter in a bowl until everything is wet and crumbly. Divide the streusel between all the muffin cups. It will seem like a lot, but use it all.
Bake at 375˚F for 15 to 20 minutes, until a toothpick comes out of the center clean. Allow to cool completely. Store any leftovers in an airtight container up to 3 days.