For super thick Rice Krispie Treats, grease an 9x9-inch baking pan with nonstick cooking spray. For regular Rice Krispie Treats, grease a 9x13-inch baking pan with nonstick cooking spray. You can also line the pan with parchment paper to make it easier for them to come out of the pan. Set pan aside.
Place the butter in a large pot and melt over medium heat, stirring often, until it foams, smells nutty, and browns. This will take about 5 minutes.
Add the 16 oz bag of marshmallows and stir until the marshmallows are completely melted.
Remove the pan from the heat and stir in the Biscoff cookie butter, vanilla extract and sea salt. Add the Rice Krispies and chopped Biscoff cookies; gently stir until well coated with the marshmallows. Stir in the remaining 2 cups of mini marshmallows.
Transfer mixture to the prepared pan. Lightly press the mixture into the pan, using a greased spatula. You can also lightly grease your hands and press the mixture into the pan. Don’t press too firmly or the treats will end up hard and dry. Sprinkle a little flaky sea salt on top, If desired.
Let the Rice Krispie Treats sit for about 1 hour. Cut into squares and serve.
Cover pan with plastic wrap or put the treats in an airtight container and store on the counter for up to 3 days.