- Butter an 8-inch (20 cm) glass square pan and line it with a strip of parchment paper, letting it hang over two sides.
- In a large pot over medium heat, melt the butter. Add the marshmallows and vanilla (see note). Cook while stirring until the marshmallows have melted. Remove the pot from the heat.
- Add the cereal to the pot and mix well. Spread the mixture out in the prepared pan, pressing down firmly into an even layer. Cover with plastic wrap. Let cool for 30 minutes at room temperature.
- Unmould the mixture from the pan and place on a work surface. Cut into 16 squares. The squares will keep for 1 week in an airtight container at room temperature.
You can replace the mini marshmallows with the same quantity of larger marshmallows.
If you are using mini marshmallows, you can set aside 3 cups (150 g) of them and add at the same time as the cereal. They will stay whole and give the squares a soft, gooey texture.