1 pound trimmed rhubarb (cut into 1/2-inch slices)
3/4 cup (packed) light brown sugar
1 teaspoon grated orange zest
3 cups unbleached all-purpose flour
2 teaspoons baking powder
1/2 cup Sugar (plus 2 tablespoons as garnish)
1/4 teaspoon salt
1/2 cup chilled unsalted butter (cut into small cubes)
1 teaspoon grated orange zest
3/4 cup buttermilk
1 large egg (separated)