Make streusel topping: In a medium bowl, whisk together the flour, sugar, cinnamon, salt and orange zest. Cut in the butter using a pastry blender or fork, until fine crumbs form. Using your hands, squeeze together the mixture to form large clumps. Store, refrigerated, until ready to use.
Muffins
Make the muffins: Heat oven to 350ºF. Line two standard muffin tins with 22 muffin papers.
In a large bowl, whisk together the flour, baking powder, and salt; set aside. In the bowl of an electric mixer fitted with the paddle attachment, mix together the butter and sugar.
Add egg, egg yolks, vanilla and orange zest. Mix until well combined and lightened in color. On low speed, gradually add in flour mixture, alternating with the milk, and mixing between each addition, beginning and ending with flour.
Remove bowl from mixer and gently stir in rhubarb. Divide batter between muffin papers. The papers will be almost full. Distribute streusel topping over muffins evenly, gently pressing it into the muffin batter.
Bake, rotating pans half-way through baking, until light golden and cooked through, 30 to 32 minutes. Test muffins with a toothpick, inserting it and checking to see if it comes out clean or wet. If it comes out clean, or just with a little crumb on it, the muffins are done.
Let muffins cool in pan about 15 minutes and then remove to a cooling rack.
Cool completely before storing in an airtight container at room temperature, for up to 4 days.
Notes
Nutritional numbers are for one muffin. • You can change the spice and add ground ginger in place of the ground cinnamon. • You can swap the fruit and add diced peaches, blueberries or strawberries.