Preheat oven to 325°. Line a springform pan with parchment paper (bottom only). Attach ring of pan to bottom and grease lightly with baking spray. In a large bowl, whisk flour, zest, baking powder, soda and salt; reserve. In small mixing bowl, add eggs, sugar, ricotta, orange juice, vanilla and ½ c marmalade and whisk until smooth. Gently fold wet mixture into dry ingredients until combined. Drizzle in butter and stir only until combined well. Pour into prepared pan; smooth top, then gently tap pan to remove any air pockets. Bake until golden brown and pick inserted in center of cake comes out clean, 50-60 min (if browning too quickly, cover with foil until center is set). Let cool in pan 15 min before removing ring. Cool to room temp until ready to serve. Meanwhile, slice peel from oranges; cut segments away from membrane or halve and slice thinly. In small saucepan, warm 1/3 c orange marmalade with ¼ c of water and toss gently with citrus slices. Pour mixture over cooled cake. Serve immediately with candied citrus garnish, if desired.