Preheat the oven to 350F. Line a 6 cup muffin pan with paper liners. For the topping, in a small bowl, combine 1 tsp. of the sugar and 1/2 tsp. of the orange zest, pressing the zest into the sugar with the back of the spoon. Set aside.
In a mixing bowl, stir together the flour, baking powder, salt and remaining orange zest. Stir in the remaining sugar. In a small mixing bowl, combine the egg, milk, melted butter and orange juice concentrate. Add all at once to the flour mixture and stir just until moistened.
Divide the batter equally among the prepared cups. Sprinkle the tops with the sugar orange mixture. Bake until the tops spring back when lightly touched, 20-25 minutes. Cool on a wire rack for 10 minutes, then turn out and cool completely. You can spoon a dollop of orange marmalade into the center of the batter in each cup before baking. You can also top with chocolate frosting.