Chop pistachios and set aside 1-2 tablespoons pistachios and 1 cup of marshmallows. (Optional: if using large or jumbo marshmallow, cut into smaller pieces)
Line a 9x9 pan with parchment paper.
In a large pot or Dutch oven, melt butter over medium heat. Swirl the pan occasionally, this will ensure the butter is evenly cooked. As the butter melts, it will bubble and foam. The color will change from yellow to a toasty brown. When you smell a nutty aroma, turn the heat off. You should see little golden brown bits at the bottom if looks black either try straining them OR you may have to throw it away and start over.
Immediately, add in your majority of marshmallows, vanilla and salt.
Turn the heat back to low-medium and stir the mixture until the marshmallows are completely melted.
Turn heat off and remove the pan from heat. Add in cereal and majority of pistachios. Using a rubber silicone spatula, stir until evenly combined.
Add in reserved marshmallows. (If using the cut up marshmallow, separate as they will try to stick together.)
Stir until marshmallows are partially melted but not completely combined. You want pockets of marshmallow goo in every bite.
Transfer the mixture to the prepared pan and using the butter wrapper or parchment paper and gently press into an even layer in the prepared baking pan.Top with leftover pistachios and sprinkle and flaky sea salt.
Let cool at room temperature for at least an hour. Then, remove treats from pan and paper onto a cutting board. Cut into squares/rectangles and enjoy. (Optional: trim/cut the edges to cut the treats into clean squares/rectangles).