Preheat oven to 375F. Line a 12-cup muffin tin with paper cups.
In a large bowl, combine the flour, sugar, baking powder, cinnamon, baking soda, and salt and whisk to blend.
In a medium bowl, whisk together the sour cream, melted butter, molasses, eggs, vanilla, the zest of one orange and orange flower water until smooth. Gently stir the sour cream mixture into the dry ingredients until the batter just comes together; do not overmix. Gently stir in the diced rhubarb. The batter will be thick.
Divide the batter among the muffin cups. The batter should mound a bit higher than the tops of the cups.
Make the topping: In a small bowl, combine the flour, sugar, and butter and rub together with your fingers. Sprinkle over each muffin.
Bake the muffins until they’re golden brown and spring back when gently pressed for about 18 minutes. Transfer to a rack and let the muffins cool in the pan for 5 to 10 minutes. Carefully lift the muffins out of the pan and let them cool. Serve warm.