Mix the flour, granulated sugar, baking powder, and salt in a bowl. Mix in the pieces of butter until the mixture is crumbly. This process is called "sanding".
Add the whipping cream and mix to form a dough.
Wrap the dough in plastic wrap and refrigerate for 1 hour or up to overnight.
Heat oven to 350 degrees.
Mix the pistachios and brown sugar in a food processor to finely chop. Add the egg yolk and rose water until it becomes a paste. Set the filling aside.
On a floured surface, roll out the chilled dough into a 12"x10" rectangle. Spread the filling over the surface of the dough.
Roll the dough into a log, using a bench scraper or metal spatula to lift the dough from the surface so it doesn't tear.
Cut on a diagonal line into 3/4-inch slices cut side down and place on a parchment-lined sheet pan, about 1 1/2 inches apart.
Bake the cookies until they are firm to the touch, about 14-15 minutes. Transfer to a wire rack and let cool. Dust with powdered sugar while they are still warm.
Store in a covered plastic container for 3-5 days.
Makes about 16 cookies