Line an 8x8-inch pan with parchment paper or aluminum foil. Spray the paper/foil with baking spray. Lightly spray a piece of parchment or wax paper with baking spray. Set both aside.
Measure 8 cups of the marshmallows into a large bowl. This will make it easier to mix them into the melted butter later in the recipe. Add the remaining 3 cups of marshmallows to a separate small bowl.
Melt the butter in a large pot (an 8-quart pot works well here) over medium-low heat. Stir in the salt and vanilla extract.
Turn the heat to low and mix 8 cups of the marshmallows (the ones you measured into the large bowl) into the butter mixture. Stir with a silicone spatula until almost completely melted and the butter is incorporated. Take the pot off the heat and mix the puffed rice cereal and about 3/4 of the sprinkles into the melted marshmallows just until a cohesive mixture starts to form. Immediately mix in the remaining 3 cups of marshmallows until fully combined. You should have lots of intact marshmallows studded throughout the rice mixture.
Transfer the marshmallow-rice mixture to the prepared pan and using the sprayed side of the parchment/wax paper, lightly but firmly press the mixture into an even layer in the pan. The sprayed paper will prevent you from making a sticky mess with your hands or the spatula. Try not to press too hard or you'll compress the treats and make them rock hard.
Distribute the remaining sprinkles over the top of the treats and press them in lightly to help them stick to the top of the treats. Cover the pan loosely with aluminum foil and allow the treats to cool and set up for about 1 hour.
Pull the treats in the parchment paper/foil up and out of the pan and cut into 16 squares.Storage: These treats taste the best on the day they are made. Wrapped tightly in plastic wrap and stored at room temperature, they will last for 2 days, max.
Please keep in mind that the nutritional information provided below is just a rough estimate and variations can occur depending on the specific ingredients used. Recipe Note: Just about any type of mini marshmallows will work for these treats - don't get stuck on finding a brand name, especially if your grocery store doesn't carry national brands. Vegan mini marshmallows should work fine here too. Brown or white puffed rice cereal will work fine for these treats as well. And as for the sprinkles, GO NUTS! Customize your sprinkles for a holiday, birthday party theme, or someone's favorite color. The possibilities are endless!