Melt butter over medium heat, add flour in batches and brown in about 15 min. Pour about 900 ml of water, add sugar and bring to a boil.
Boil about 100 ml of water, melt saffron + cardamom, add rose water and stir into flour mixture. Simmer a little on low heat until the mixture has a creamy consistency. Pour into a large enough bowl, garnish with pistachios and bring to the table.