Preheat oven to 180°C.
Butter and line a 30x20cm rectangular baking tin with baking parchment.
Stir flour, sugar, spice, bicarbonate of soda, sultanas, and salt in a large bowl.
Beat eggs with caster sugar, then incorporate melted butter, orange zest, juice, and syrup.
Mix the wet and dry ingredients until combined and fold in the pumpkin and beetroot.
Bake the batter in the tin for 30 minutes until golden-brown and springy.
Beat cream cheese, butter, and 33g of icing sugar until smooth and set aside.
Whisk egg whites and remaining icing sugar over a pan of simmering water until thick and glossy.
Whisk the cooled egg white mixture until firm and gradually incorporate the cream cheese mixture.
Cool the cake for five minutes and then transfer it to a cooling rack.
Heat orange juice and syrup, then drizzle over the warm, pricked cake.
Spread the frosting over the cooled cake, decorate with orange zest, and serve.