Preheat the oven to 400 degrees F. Lightly grease 2 small muffins tins (with 24 (1 3/4-inch) wells), and set aside.
In a large bowl, sift together 1 1/2 cups of flour, the sugar, baking powder, and salt.
In a bowl, beat together the egg, 8 tablespoons of the butter, the milk, orange juice concentrate, and 2 teaspoons of the zest. Add the wet ingredients to the dry, combining just until moistened and being careful not to overmix. Divide the batter among the prepared muffin tins, filling each halfway.
To make the crumble topping, in a small bowl, combine the remaining 2 tablespoons of flour, the chopped pecans, the brown sugar, and the remaining 1 tablespoon of melted butter and 1/2 teaspoon of grated zest. Mix to combine and sprinkle 1 teaspoon on top of the batter for each muffin. Bake until the muffins are set and golden brown and a tester inserted into the middle comes out clean, 14 to 16 minutes.
Remove from the oven and let sit for 5 minutes in the tins, then cool on wire racks. Serve with orange-butter on the side.
Yield: 2 1/2 dozen