Butter and line a traybake or small roasting tin, about 20 x 30cm. Heat oven to 180C/160C fan/gas 4. Gently melt the butter in a large saucepan, cool for 5 mins, add the sugar, vanilla and eggs, then beat until smooth with a wooden spoon. Stir in the flour, coconut and ¼ tsp salt. Stir the carrot and mixed spice into the mix. Bake for 30 mins.
Meanwhile, evenly mix 85g more coconut with 25g light muscovado sugar and 25g melted butter. Smooth this over the cake, then bake for 10 mins more until golden and a skewer inserted comes out clean. Cool, then cut into squares.