For the Frosting:
Make the frosting. Drain cashews and place into a blender with yogurt, powdered sugar and cacao powder. Process until creamy and smooth. Transfer the frosting into a bowl, cover with a plastic wrap and set aside for later. Preheat oven to 180°C and grease a baking tin.
In a bowl sift flour, baking powder, baking soda, salt and chocolate powder together. Set aside. In a separate bowl using a hand electric mixer beat oil and sugar together.
Beat in eggs and yogurt. Beat in vanilla, all spice and dry ingredients. Fold in carrots and walnuts, stir until combined. Spoon the mixture into the prepared baking tin and bake for 35-40 minutes or until a toothpick inserted into the middle comes out clean. Remove from the oven and let cool for 10 minutes in the baking tin, before removing. Transfer to a wire rack to cool completely. Frost the cooled cake and decorate with chopped walnuts.