Preheat the oven to 180° Celsius

Spray a 23cm springform tin and line the bottom

Line the bottom of the tin with the grapefruit

Place the eggs, honey and fructose into a bowl of a stand mixer and whisk until light and frothy

Add the buttermilk and marmalade and continue whisking to combine

Slowly pour in the oil while you continue whisking

Sift the flours, bicarbonate of soda, baking powder and spices into a mixing bowl and whisk to combine

Gently fold in the dry ingredients until the flour has been completely incorporated

Scrape down the sides and then carefully pour the batter into the tin

Bake for 40 minutes

Remove from the oven and place the tin onto a wire rack and leave to cool for 40 minutes

Remove the springform and invert onto a plate, grapefruit side up

Slide a knife around the edge of the base to remove

Carefully remove the baking paper and serve