Pre-heat the oven to 160°C/Gas 3. Grease a 22cm cake tin (one with a removable base) and line it with baking parchment.In a large mixing bowl, cream the butter and sugar together. When the mixture is thoroughly combined, beat in the eggs.Place the cardamom pods in a mortar and work with a pestle to get the seeds out of the pods. Discard the pods and grind the seeds to a fine powder. Add them to the cake mixture, along with the flour, ground almonds, lemon zest and juice, rose water, baking powder and salt. Mix well.Pour the mixture into the cake tin and bake in the oven for 45 minutes. To check if it is ready, stick a fork in the middle of the cake — it should come out dry.Towards the end of the cooking time, make your drizzle topping. Place the caster sugar, lemon juice and rose water in a small pan over a low heat and heat until the sugar melts.Remove the cake from the oven and place it on a wire rack. Poke holes all over the top of the warm cake and drizzle over the syrup.When the cake is completely cool, make the icing by combining the icing sugar, lemon juice and a few teaspoons of water until you have a smooth, thick icing. Spoon the icing over the cake and finish with a sprinkling of sliced pistachios and, if you like, rose petals.