Preheat oven to 375 F. In a large bowl mix the flour, lemon zest, baking powder, sugar and salt. Add melted butter and beat with a hand mixer until smooth. Add in the eggs, water, yogurt and vanilla. Mix until well combined. Spread the mixture into a greased 11-in. x 13-in. baking pan. Bake for 18-20 minutes or until a toothpick inserted near the center comes out clean. Meanwhile, make the lemon syrup, by boiling the water and the sugar together, then add the lemon juice. Boil for about 3 minutes. Remove from the heat. Let to cool and mix with a rum. Remove the cake from the oven and drizzle with the syrup. Leave to cool. In a medium bowl, beat the cream cheese, mascarpone, lemon juice and confectioners' sugar until smooth. Stir in the whipped cream. Spread the frosting over the cake. Store in the refrigerator.