Preheat the oven to 180ºC, fan 170ºC, gas 4. Line a 12-hole muffin tin with muffin cases.
In a large bowl, combine the soft brown sugar, flour, cinnamon and bicarbonate of soda. Make a well in the centre and add the eggs, oil, carrot and orange zest. Mix well and add the pineapple and walnuts. Stir to combine, then spoon into the muffin cases.
Bake for 20 minutes or until a skewer poked in comes out almost clean. Allow the cupcakes to cool before decorating with buttercream icing.
For the icing, beat together the icing sugar and softened butter. Divide among several bowls and add crushed blueberries, raspberries or strawberries. Pipe a design on to each cake and sprinkle with caster sugar that's had colouring stirred through.