To make the filling: Place the pistachios, almonds, powdered sugar, and cinnamon in a food processor. Pulse several times, until the nuts are very finely chopped and the sugar is evenly mixed in with the nuts. Add the rosewater, honey, and egg to the food processor and pulse several more times, until the filling is evenly damp. It should be a thick, sticky mixture, almost paste-like. Set aside.
To make the cookie dough: Cream the butter and the sugar together on medium speed (or vigorously by hand) until light and fluffy. Add the egg and beat until it is evenly incorporated. Add the vanilla to the dough and beat to combine. Add the flour and baking soda and beat until just combined (don’t overmix). The dough should be smooth and easy to roll into small balls.
To assemble and bake cookies: Preheat the oven to 350F and line a cookie sheet with parchment paper. Roll the cookie dough into small balls, slightly smaller than the size of a ping pong ball. Space the balls evenly on your prepared cookie sheet, then use your thumb to make a deep impression in the center of each ball. Fill the thumbprints with the nut mixture, pressing the filling gently against the sides of the cookie to help it adhere. Bake the cookies until they are just starting to turn golden brown on the top, about 15 minutes. Remove from the oven and let cool.
If you’d like to decorate with a white chocolate drizzle, place the white chocolate in a metal bowl. Bring a small pot of water to a simmer, then place the metal bowl on top of the pot. Gently melt the chocolate, using a spatula to stir it and encourage even melting. As soon as all the chocolate is melted, remove the bowl from the heat (use pot mitts – the bowl may be hot!) and use the spatula or a spoon to drizzle white chocolate on top of the cookies. Let chocolate harden before serving.