Line a 20 cm (8 inch) round cake tin with nonstick baking paper. Place the butter, sugar and orange rind in a large bowl and beat with a hand-held electric whisk until light and fluffy. Add the ground almonds and eggs and beat well. Stir in the orange juice, polenta and baking powder and mix until well combined.
Spoon the mixture into the prepared cake tin and place in a preheated oven, 180ºC (350ºF), Gas Mark 4, for 20–25 minutes until risen and firm to the touch.
Meanwhile, place all the syrup ingredients in a saucepan and bring to the boil. Reduce the heat and simmer for 2–3 minutes.
Spoon the syrup over the cake in the tin, then turn out and serve in slices.