Whisk butter, sugar, eggs, juice, and zest in a bowl over simmering water until thickened.
Cool the curd to room temperature then refrigerate to set.
Boil sliced grapefruit peel in water, drain, and simmer in 75ml water and sugar for 10 minutes.
Coat the peel in sugar and leave to dry on a wire rack.
Preheat the oven to 180°C and grease two 20cm tins.
Mix cake ingredients in an electric mixer and divide batter between tins.
Bake for 30 minutes, cool briefly in tins, then transfer to a wire rack.
Boil 75ml grapefruit juice and sugar for five minutes to create a thickened syrup.
Pierce warm cakes and drizzle with the prepared syrup before cooling completely.
Whisk mascarpone, cream, and icing sugar until thick and airy.
Spread a layer of cream and curd onto the bottom cake layer.
Top with the second cake and cover evenly with the remaining cream.
Pipe cream decorations around the edges and garnish with candied peel.