Preheat the oven to 350 degrees F and grease and lightly flour 2 (8 or 9-inch) round cake pans.
In the bowl of an electric mixer, cream the butter and zest. Add the sugar little by little, until mixture is very light and fluffy. Add the eggs, one at a time, beating well after each addition. Sift the flour and baking powder together and add to butter mixture in three stages, alternating with 1 tablespoon of the rum after each addition. Mix on low speed until just incorporated. Divide the mixture between the two prepared cake pans, and bake for about 25 minutes, or until the cake is golden brown and springs back when lightly touched. Allow the cakes to cool in the pans for a few minutes, then turn out onto cooling racks lined with waxed paper.
In a small saucepan, combine the ortanique juice and guava jelly and heat until jelly is softened and combined with the fruit juice. Remove from the heat and stir in the rum.
Use a toothpick to make small holes all over the bottoms of the cakes, then brush with 1/3 of the glaze each. Once the cakes are completely cool, combine the heavy cream and vanilla extract in a mixing bowl and beat to soft peaks. Add the confectioners' sugar and continue to beat until cream holds stiff peaks. Place each cake layer on a serving platter (glazed side up) and spread the whipped cream evenly on top of the cakes. Decoratively arrange the ortanique segments around the edges of the top of the cakes, then brush the segments with the remaining glaze. These cakes are best when eaten the day they are made, just after being assembled.
*Ortanique is a hybrid of a tangerine and a sweet orange. If you are unable to find ortaniques, simply substitute equal parts of tangerine and any sweet orange.
Yield: 2 cakes, serving 10 to 12