Celeriac-and-nut cake with date butter

Preheat the oven to 180°C. Place the oranges and lemons in a saucepan and cover with water. Simmer over a medium- low heat for 25 minutes. Drain and refresh the water, repeating the process twice. This process will remove any bitterness. Set aside to cool.
Grease and line a 23 cm springform cake tin.
Roughly chop the cooled fruit (do not peel), removing the white pith in the centre and any seeds.
Blend the citrus is a food processor to form a purée. Measure out 11⁄3 cups of purée and set aside.
Using an electric mixer, beat the eggs until light and creamy, then gradually add the sugar and baking powder. Continue beating until thick and pale in colour. Gently fold in the ground hazelnuts and the citrus purée until well combined. Stir in the cardamom seeds.
Pour the mixture into the cake tin and bake for 50 minutes, or until a skewer inserted comes out clean.
Cook's note: You can also use almond flour in this recipe.