Preheat the oven to 180°C/Gas Mark 4. Grease the holes of a Lakeland Bakeware 12-hole loose-based mini loaf tin with a little of the melted butter.
Add the orange juice, beaten eggs and vanilla extract to the remaining melted butter, stirring to mix.
Sift the flour, salt, bicarbonate of soda and baking powder into a large mixing bowl. Stir in the sugar, carrots and courgettes. Add the melted butter mixture and stir together well, but do not beat or overmix.
Spoon the mixture into the prepared tins. Bake for 20-22 minutes, or until a fine skewer inserted into the centre of the cakes comes out clean. Cool in the tins for 15 minutes, then turn out carefully. Cool completely on a wire rack.
Mix together the icing sugar and orange juice. Drizzle over the cooled cakes, then scatter a little orange zest on top.