Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
In a large mixing bowl, cream the softened butter, granulated sugar, and brown sugar together until light and fluffy.
Beat in the egg and vanilla extract until well combined.
In a separate bowl, whisk together the all-purpose flour, baking soda, and salt. Gradually add this dry mixture to the wet ingredients, mixing until just combined.
Gently fold in the Rice Krispies cereal and optional chocolate chips or sprinkles.
Scoop tablespoon-sized portions of dough and roll them into balls. Place them on the prepared baking sheet, leaving about 2 inches of space between each cookie.
Bake for 10–12 minutes, or until the edges are golden brown.
Remove from the oven and let the cookies cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.
Serve and enjoy your crispy, chewy Rice Krispie Cookies!