Preheat the oven to 425°F/220°C and line a 12-cup muffin pan with liners.
Combine flour, brown sugar, granulated sugar, and cinnamon in a bowl. Cut in cold butter with a pastry cutter or your fingers until the mixture resembles coarse crumbs. Chill the streusel while you prepare the batter. 130 g soft brown sugar, 125 g all-purpose flour, 2 tsp ground cinnamon, Pinch salt, 100 g unsalted butter
In a small bowl, combine flour, spices, salt, and baking soda and whisk. Set aside. 250 g all-purpose flour, 1 ½ teaspoons pumpkin spice*, 1 teaspoon baking soda, ¼ teaspoon baking powder, ½ teaspoon salt
In a large bowl, add the pumpkin puree, oil, Greek yogurt, white and brown sugar, vegetable oil, eggs, and vanilla. Whisk it all together. 226 g pumpkin puree, 120 g Greek yogurt, 105 g vegetable oil, 100 g granulated sugar, 100 g soft brown sugar, 2 large eggs, 1 teaspoon vanilla extract
Add the dry ingredients to the pumpkin mixture and mix them together with a rubber spatula until combined. Don't over-mix it.
Divide half of the pumpkin muffin batter evenly among 12 muffin cups. Generously pile on half of the crumb topping.
Scoop over the remaining muffin batter, followed by the remaining crumb topping. There will be a lot. Carefully and gently press it onto the batter. It’s very crumbly at this stage, and that’s completely normal.
Bake the muffins at 425°F (220 °C) for 5 minutes, then reduce the oven temperature to 350°F (180 °C) and continue baking for an additional 25 minutes, or until the tops of the muffins spring back when lightly pressed. Let them cool in the tin for 10 minutes before removing and placing on a wire rack to cool completely.
In a small bowl, whisk together the powdered sugar, milk and vanilla until smooth. 60 g powdered sugar, 1/2 -1 Tbsp milk, 1/4 tsp vanilla paste
Drizzle over the cooled muffins.