In a 7 cup or larger food processor fitted with a steel S blade, add the protein powder and rolled oats. Blend for 1 minute, until oats are powdery and fine.
Add pistachio butter, honey, tahini, pistachios, melted coconut oil, and chocolate to the food processor. Blend for another 30 seconds, or until mixture is well combined.
Fold some of the mixture in your hand; it should form a ball and not crumble. If the mixture is crumbly, add 1-2 teaspoons of water and process again.
Line a 9 x 5 inch loaf pan with parchment, and press the protein bar mixture into the pan until smooth across the top. Then chill for 2 hours (or freeze for 30 minutes). Once it has set, slice into 12 even bars.
OPTIONAL TOPPING: Slowly melt the ⅓ cup of dark chocolate with the coconut oil (30 seconds in the microwave, then stir). Once the chocolate is smooth, pour it over the mixture pressed into the lined pan. Tilt the pan back and forth until the chocolate evenly covers the top of the bars. Sprinkle with pistachios and sea salt.
Place pan in the fridge and chill for two hours or in the freezer for 30 minutes. Once set, remove and slice into 12 equal bars.