Prepare the honey syrup in advance.
Thaw the phyllo dough according to the instructions to the package. Preheat the oven to 325F degrees.
Use a food processor to grind the pistachios. Add the ground cardamon and mix together. Set aside.
Keep the phyllo dough covered with a damp (not wet) towel while working with it to prevent it from drying out. If it dries out too much it will be crumbly and difficult to work with.
Generously butter a 9x13" baking dish. Lay a sheet of phyllo dough down and brush with melted butter. Continue until you have 10 sheets of buttered phyllo dough layered in the dish.
Add ⅕ of the ground pistachios on top of the phyllo dough. Layer another 5 buttered sheets of phyllo over the pistachios. Continue layering pistachios between 5 sheets of phyllo dough until pistachios are gone.
Top the last layer of pistachios with 10 sheets of phyllo dough.
Use your hands to press down lightly to remove any air bubbles, then brush with the last layer of butter.
Bake for 60-70 minutes, until phyllo dough is golden brown and crisp.
Remove from the oven and pour the room temperature syrup over the hot baklava. You should hear it sizzling.
Let the baklava sit for 6-8 hours before serving to allow the flavors to meld together.