Combine butter, water, milk, salt, and granulated sugar in a medium saucepan or pot over medium heat, stirring until the butter has melted. Bring the mixture to a simmer and then reduce heat to low. Add all of the flour and stir until completely incorporated. The dough should be a thick ball. Press against bottom of sides of the pan for 1 minute to cook the flour and remove from heat.
Place ball of dough into a large mixing bowl and allow to cool for a few minutes before adding eggs.
Slowly add the beaten eggs in 3 or 4 batches. Between each batch, use a spatula to mix well and observe the texture of the dough. At first it will look clumpy and curdled, but will eventually become thick and glossy. Stop when you have a thick and pipeable texture. The batter should fall easily off your spatula, but have enough strength to leave behind a "V" shaped flag. I like to save at least 1-2 tbsp of egg to use as the egg wash.
Preheat oven to 400 degrees F. Line a baking sheet with parchment paper and trace an 8 inch circle into the middle of the paper. Lightly brush the sheet with water. The water helps create extra humidity in the oven, which helps with puffing.
Fill a piping bag with pastry dough and cut off the tip to create a 1/2 inch opening. Pipe one ring of pastry along the 8 inch circle. Pipe a second ring along the inside of the first ring, making sure that they are touching. Pipe a third ring on top, in between the first two rings.
Mix the leftover egg from the pastry dough with 1 tsp milk to create an egg wash. Brush on the surface of the pastry, then sprinkle with crushed pistachios.
Bake for 15 minutes and then, making sure not to open the oven door, reduce oven temperature to 350 degrees F and bake for 10 - 15 more minutes until golden brown
Remove from oven and let cool completely on a wire cooling rack