Heat oven to 180°C/160°C fan/gas mark 4. Butter and line a 30 x 20cm baking or small roasting tin with baking parchment.
Put the flour, sugar, spice, bicarbonate of soda and salt into a large bowl and stir to combine. Beat the eggs into the melted butter, stir in the orange zest and juice, then mix with the dry ingredients till combined.
Stir in the pumpkin flesh. Pour the batter into the tin and bake for 30 mins, or until golden and springy to the touch.
To make the frosting, beat together the cheese, butter, icing sugar, orange zest and 1 tsp of the juice till smooth and creamy, then set aside in the fridge.
When the cake is done, cool for 5 minutes then turn it onto a cooling rack. Prick it all over with a skewer and drizzle with the rest of the orange juice while still warm. Leave to cool completely.
Beat the frosting briefly to loosen, then use a palette knife to spread it over the cake in peaks and swirls.