Preheat the oven to 180°C/Fan 160°C/ Gas 4. Lightly grease a 23cm springform cake win with butter and line the base and sides with non-stick baking parchment.
Using a freestanding electric mixer fitted with a paddle attachment, or an electric hand mixer and a large bowl, beat the oil, sugar, eggs, orange zest and juice, treacle and vanilla seeds together until fully combined.
Sift the flour, baking powder, salt, cinnamon, nutmeg and mace together over the mixture. Sprinkle over the black pepper, then scatter over the grated carrots, sultanas and chopped walnuts. Gently fold everything together with a spatula until evenly combined but be careful not to overwork the mixture. The batter should be fairly soft and a little runny.
Pour the mixture into the prepared cake tin and bake for about 40 minutes, until nicely golden and springy to the touch. To test, insert a skewer into the centre of the cake: it should come out clean. Leave to cool in the tin for 5-10 minutes, then turn out onto a wire rack and carefully remove the parchment. Leave the cake to cool completely.
To make the icing, using either a freestanding mixer with the paddle attachment or electric hand mixer and large bowl, beat the mascarpone and butter together until smooth. Beat in the coconut, then sift the sugar over the mixture and beat until the icing is smooth.
Using a palette knife, spread the icing all over the top and sides of the cake. To finish, sprinkle on the toasted coconut. Serve cut into generous slices.