Preheat the oven to 170°C.
Grease a 20cm round tin with butter and line the base with parchment.
Mix the ground almonds, polenta, flour, and bicarbonate of soda in a bowl.
Beat the butter until smooth using a mixer, then incorporate the honey and grapefruit zest.
Beat in the eggs and the dry ingredients and bake the batter for 30 minutes.
Boil the citrus juices and honey until the syrup reaches 105°C.
Prick the cooled cake and drizzle with half of the prepared syrup.
Peel slices of skin from the grapefruit and orange using a potato peeler.
Boil the peel in water for 2 minutes and repeat three times to remove bitterness.
Simmer the peel in the remaining syrup for 10 minutes until translucent.
Reduce the maple syrup by boiling, let it cool, and mix into the beaten mascarpone.
Spread the maple cream over the cake and garnish with the candied peel.