Prepare your chestnuts by preheating oven to 200 degrees celsius. Use a sharp knife to cut a cross in the top of the chestnuts, place them in a baking pan and roast for 20 minutes in the oven. Remove shell.
Use the same baking pan and heat from oven to give the walnuts a little roast, approximately 5 - 7 minutes or until they look golden.
In a high speed blender add all walnuts and chestnuts and blend until very fine texture. Then add oil, salt, water and pulse until the perfect creamy texture is reaches. Season to taste.
Keep the butter in a jar in the fridge for up to 2 weeks. Enjoy!