Prep pan. Line a 9x13 pan with parchment paper or wax paper. Make sure the parchment paper comes up the long sides of the pan to form a lip. Clip the sides with binder clips so the parchment paper doesn't move.
Make marshmallow mixture. In a large stock pot, melt butter over medium heat. When the butter is completely melted, add 80% of the mini marshmallows and the vanilla, if using. (Reserve 20% of the mini marshmallows to add later.) Stir into the butter as it melts slowly, about 2-3 minutes.
Add crispy rice cereal. When the marshmallows are melted into one large gooey mass, turn off the heat. Add the crispy rice cereal. Stir gently with a spatula until well combined, about 1-2 minutes. Fold the mixture gently so the marshmallow mixture is evenly dispersed.
Add reserved mini marshmallows. Add the reserved marshmallows and mix gently. The residual heat will melt the reserved marshmallows into gooey chunks. Immediately transfer to the prepared pan. Press gently into the pan with a spatula. Do not press too hard!
Melt chocolate. Add chocolate chips to a small, heat-proof bowl. Microwave for 60 seconds, then 30-second bursts until the chocolate has melted. Stir with a spoon until smooth. (If you don't have a microwave, use a double boiler to melt the chocolate chips.)
Add chocolate layer. Add the chocolate to the rice krispies. Spread evenly over half of the pan, in one layer, making sure to reach the corners.
Add sprinkles and flaky sea salt. Immediately add the sprinkles. Add the flaky sea salt, too. Spread evenly all over.
Chill. Transfer to the fridge for 20-30 minutes to chill. The chocolate will become firm. (Do not leave in the fridge for too long or the marshmallow will harden, too!). Or, leave at room temperature for 1-2 hrs.
Serve. Remove the binder clips. Grab the edges of the parchment paper and remove from the pan. Using a bread knife (or serrated knife) cut into 32 squares. Enjoy!
*Leftovers can be stored at room temperature in an airtight container for 3-4 days.