Melt 6 tablespoons 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. Immediately reduce the heat to low. If the pot is very hot, briefly remove it from the burner to keep the butter from burning.

Add the 10 ounces of marshmallows. Stir over low heat until just melted, incorporating the browned butter bits from the bottom of the pot. Avoid cooking the mixture longer than necessary.

Remove the pot from the heat. Stir in the creamy peanut butter, vanilla extract, and salt until smooth and evenly combined.

Add 5½ cups of Rice Krispies cereal and gently fold until evenly coated. Add up to another ½ cup of cereal if you prefer firmer treats. For softer bars, stick with the smaller amount.

Immediately transfer the mixture to the prepared pan. Lightly grease a spatula or your hands, then gently spread the mixture to the edges. Press just enough to create an even layer without packing it tightly.

Let the treats cool and set at room temperature, about 1 hour. Lift them out using the parchment paper and cut into 9 large squares or 16 smaller squares.