Brown the butter. Melt ½ cup unsalted butter in a large, heavy-bottomed pot over medium-low heat. Continue cooking, stirring frequently, as the butter begins to foam and froth. Once the foam settles slightly and you see golden-brown flecks on the bottom of the pot, remove the pot from the heat or reduce the heat to low. The butter should smell rich and nutty. This generally takes about 4 to 6 minutes.

Melt the marshmallows. Add the 10 ounces of marshmallows, 1 teaspoon vanilla extract, and ¼ teaspoon salt to the browned butter. Stir over low heat until the marshmallows are almost completely melted, with only a few small lumps remaining.

Add the peanut butter. Remove the pot from the heat. Stir in ½ cup creamy peanut butter until the mixture is smooth and evenly combined.

Fold in the cereal. Add 5½ cups of Rice Krispies cereal and gently fold until evenly coated. If the mixture seems especially loose or gooey, add up to another ½ cup of cereal. For softer, chewier treats, avoid adding more than necessary.

Add the extra mini marshmallows. Let the cereal mixture cool in the pot for about 30 to 60 seconds. Quickly and gently fold in 1½ to 2 cups of mini marshmallows. Some should soften and become stretchy, while others remain mostly whole.

Press into the pan. Immediately transfer the mixture to the prepared pan. Lightly grease your hands or the back of a spatula, then gently spread the mixture to the edges. Do not pack it down firmly, or the treats may become dense and hard.