1.Whisk together the flour, sugar, and cinnamon. Add the butter, and mix in using your fingers to rub the butter into the dry ingredients. Set aside.
2.Preheat oven to 375°F. Line a muffin tin with 12 paper liners.
3.In a medium bowl, mix together the flour, baking powder, baking soda, and salt.
4.Spoon avocado into a stand mixer and beat until almost smooth. Add sugar and beat until well blended.
5.Add the egg, beating until completely combined. Add the vanilla and the yogurt and mix well.
6.Put the flour mixture into a sifter and sift half of the mixture into the batter and mix until just combined. Sift in the remaining flour and mix until just blended. Gently fold in the blueberries.
7.Using a spoon or an ice cream scoop, divide the batter among the 12 cups. Sprinkle the streusel topping over the batter in the 12 cups, dividing evenly.
8.Bake for 25-30 minutes, or until a wooden tester comes out clean. Let cool in the pan on a rack for 5 minutes before removing.
9.Serve warm or at room temperature.
Serving Suggestion: Serve warm with a little butter or your favorite jam.