Melt the butter or shortening in a saucepan, and stir in the flour gradually, using a wire whisk to prevent lumps.
Stir it over medium heat until golden brown in color. Remove from heat.
In another saucepan, boil the sugar and water together until the sugar is dissolved, then add the rosewater and saffron.
Stir the syrup into the browned flour and keep stirring until the mixture thickens.
If too thin, cook it briefly over a medium heat, being careful not to burn.
When thickened, press the halvah into a plate, smoothing the top with the back of a spoon.
Cool before serving, and garnish with the chopped pistachios.