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Heat oven to 180C/160C fan/gas 4. Grease and line a 20 x 30cm traybake tin with baking parchment. Mix together the flour, baking powder, bicarb, spice and sugar in a big mixing bowl. Stir in the grated carrots, sweet potatoes and sultanas. In a jug, whisk together the eggs, rapeseed oil, agave syrup and juice from 1 orange. Tip the wet ingredients into the bowl and stir to combine, then scrape into the tin. Bake for 25-30 mins until a skewer poked in comes out clean. Prick all over with a skewer and drizzle over the remaining orange juice. Cool in the tin.
Once cool, make the icing. Stir the quark with a spoon to make it a bit smoother, then fold in the fromage frais, icing sugar and orange zest. Spread all over the cake and slice into squares to eat.