How to Make It
Combine first 4 ingredients in a heavy saucepan; cook over medium-high heat, stirring constantly, 5 minutes.
Dip orange sections, 1 at a time, into syrup, and cool on parchment paper.
Beat egg whites, cream of tartar, and salt at high speed with a mixer until foamy. Add 1 1/2 cups sugar, 2 tablespoons at a time, beating until stiff peaks form (2 to 4 minutes). Fold in vanilla and flour, 1/4 cup at a time.
Line in a 15- x 10-inch jellyroll pan with parchment paper; spread marmalade over paper. Pour batter over marmalade.
Bake at 325° for 25 minutes; cool in pan on wire rack. Sprinkle with powdered sugar. Invert onto a baking sheet; peel off paper. Cut cake with a 2 1/2-inch round cutter; serve with orange sections.