In a medium bowl, combine brown sugar, all purpose flour, cinnamon, and a tiny pinch of salt. Pour melted butter in and mix with a fork until it is all combined and the texture of crumbly, wet sand. Set aside.
Preheat oven to 425° Use a jumbo muffin pan, place tulip liners in each cup of a standard size cupcake pan, or use squares of parchment paper to make your own jumbo liners (see post for details)
Place greek yogurt into large bowl. Whisk in egg, vanilla, and melted butter. Sprinkle flour, sugar, baking powder, baking soda, and kosher salt over top of the wet ingredients and use a rubber spatula to mix until just combined. Don't over mix. Just bring together until all the dry ingredients disappear. A few lumps are okay.
Gently fold in your blueberries. If using frozen, do not defrost before adding.
Using a large scoop or measuring cup, place ⅓ cup of batter into each muffin cup (or, if you'd rather eyeball it, fill each cup ¾ full.)
Sprinkle top of each muffin with lots of streusel, about 3 tablespoons each, completely covering them. Press down a bit to encourage it to stick to the batter.
Bake at 425° for 5 minutes. After 5 minutes, without opening the oven door, turn the oven down to 350°. Bake for another 20 to 22 minutes, or until a clean skewer inserted into the center muffin comes out with few or no crumbs attached.
Cool in pan for 5 minutes, then remove to a wire rack to cool for at least another 10 minutes. If you eat them sooner, they might fall apart, but sometimes we all fall apart and we're amazing too.