Preheat oven to 425° and set rack in middle of oven. Line a 12-cup muffin tin with paper liners (recommended), or thoroughly grease and flour each muffin cup.

In a medium bowl, mix together buttermilk, oil and sugars; stir to combine. Whisk in egg and extra yolk, combining thoroughly. Stir in vanilla.

In a sifter or separate bowl, mix flour, baking powder, and salt. Sift directly into liquid mixture (or whisk in bowl and add to liquid all at once). Stir carefully until just combined (don’t overmix, which can lead to tough muffins). Fold in blueberries.

In a separate bowl, make the streusel:  combine brown sugar, flour, and cinnamon, then drizzle in melted butter. I find the easiest way to blend the mixture is to use my fingers, pinching and squishing until everything is fully combined. Or, use a fork or pastry cutter.

Fill each lined muffin cup halfway. Add about a teaspoon of streusel to each, then fill each muffin cup all the way to the top. Distribute remaining streusel evenly among muffins. This recipe should make exactly 12 standard-size muffins; should you end up with an empty cup, fill it halfway with water.

Bake at 425° for five minutes, then reduce temperature to 375° and bake for 22-25 minutes, until tops of muffins bounce back when touched and edges are golden brown. Cool and store in an airtight container for up to five days, or wrap individually and freeze for up to 3 months.