Preheat the oven to 425 degrees F and lightly mist a mini muffin pan with non-stick spray
Place the flour, baking powder, salt, and zest in a medium bowl and stir with a whisk to combine.
Place the melted butter, sugar, Greek yogurt, egg, and vanilla in a large liquid measure and whisk until incorporated.
Pour the liquid ingredients into the dry, and stir together with a silicone spatula or wooden spoon, just until ALMOST combined (you should still see streaks of flour).
Add the chopped orange segments, and fold carefully. Do not overmix.
Place about 2 tablespoons of batter in each well of the muffin tin, and top with the reserved streusel.
Bake for 3 minutes at 425 degrees F, then turn the oven temperature down to 350 degrees F (don't open the oven door), and bake for an additional 8 minutes, or until a tester comes out clean.