I read this and thought it was a fab idea! Yes, another one from my baking bible. I'm going to shorthand the recipe, partly for speed and partly because I'm not sure about reproducing in full quite so many recipes from the same book here! I did modify this one by adding the rose element, which I thought would go well with the theme.
Small cakes: (see below for large cake)
Make a basic 6oz/175g sponge mix: 3 eggs, 175g each of butter, sugar and SR flour; begin by creaming the butter and sugar, then add the eggs and finally the flour. I also added a good pinch of ground cardamom and 2tbsp coconut cream.
In the blender I whizzed up 50-60g each of pistachios, desiccated coconut and sultanas, and stirred this into the cake mix. This makes 20-24 little cakes (i.e. cake cases not muffin cases) which bake in 15-20 minutes at 180°C.
When cooled, I topped some of them with basic fondant icing (fondant icing sugar & cold water measured by eye and mixed to a stiff but still dribble-able consistency), flavoured with a couple of drops of rosewater and coloured with literally a drop of red colouring (a skewer dipped in the bottle and dotted into the icing). Then I sprinkled some crystallised rose petals on top.
One tin of these disappeared at this afternoon's fundraising tea and cake, and the other one (the original recipe was for an 8oz sponge mix - there were plenty!) will go for the same purpose at work tomorrow. I tried one (or two... *cough*) and they were yummy - just the right size, a nice nuttiness and the fragrant topping worked quite well with just a blob of icing. I'm not sure whether the cardamom adds anything, and if so, whether to dump it or increase it. Might be too much to have another flavour.
Edit (July '11): Making these again, I added 3 pods' worth of cardamom seeds, freshly crushed with a pestle and mortar. This time the flavour really came through and was well worth having (but this time there was no rose icing, so I'm still not sure about including both).
Edit 2: the recipe for pistachio cardamom whoopie pies here includes rosewater buttercream. Got to be worth a try next time!
Edit 3 (Dec '14): no buttercream, but freshly ground cardamom and rose water icing work fine together. I was a bit over-liberal with the red food colouring this time, so the icing this time was more of a shocking pink - but that was a great contrast to a sprinkling of chopped pistachios. And it earned the great nickname of Bollywood Cakes.
Edit 4 (March '17): I made an 8oz/4-egg sponge version that was baked in two 20cm tins. I topped and filled with rosewater buttercream - made with the leftover coconut cream as well as butter, it was a slightly lighter texture, and worked well with the rose. Some good raspberry jam in the middle cut through the sweetness a bit, and a sprinkle of chopped pistachios and desiccated coconut gave a clue about the contents.