In a mixing bowl, whisk together the flour, brown sugar, and salt. Drizzle in the melted butter and mix with a fork until small pebbles form. Alternatively, you can cut in softened butter with two forks or a pastry cutter to yield similar results. Set aside.
Cheesecake Filling
In a clean mixing bowl, use a hand mixer to beat the cream cheese until it is creamy. Add the sugar and blend until smooth.
Add egg yolk and beat until fully incorporated. Do not over mix.
Blueberry Muffins
Preheat oven to 425 degrees F. Line 16-18 muffin cups with cupcake liners; set aside.
In a small mixing bowl, whisk together flour, baking powder, baking soda and salt.
In a second bowl, use your mixer to cream together the butter and sugars together until fluffy. Add the eggs, Greek yogurt, and vanilla, mixing until well combined.
Slowly add the flour mixture and the buttermilk to the sugar mixer, alternating between the two.
Use a rubber spatula to fold in the blueberries, being sure not to overmix.
Place about a tablespoon of the muffin batter into the bottom of each liner. Top with a heaping tablespoon of cheesecake filling, then fill the cups almost to the brim with the remaining batter.
Sprinkle the tops of each muffin with the streusel.
Place the muffin tins in the oven and bake for 5 minutes. After 5 minutes, reduce the oven temperature to 350 degrees F and continue baking for another 12-18 minutes, or until a toothpick comes out mostly clean. Be sure to bake the muffins completely (they will be a bit dark) otherwise they will sink as they cool.
Remove the pans from the oven and allow them to cool for 5 minutes before removing to a wire rack to cool completely.