Preheat the oven to 180°C and grease 2 x 18 cm springform cake tins. Sift the dry ingredients into a bowl and add the nuts.
Whisk together the eggs, oil and maple syrup. Stir in the yoghurt and celeriac.
Pour the dry ingredients into the wet and fold to combine. Divide the batter between the tins. Bake for 30 minutes, or until a skewer comes out clean when inserted.
To make the date butter, blend all the ingredients to form a paste. Spread onto the cakes and sandwich together. Use the remainder to decorate the cake.