Line 9x9 pan with parchment paper and spray with cooking spray to prevent sticking.
Crush pretzels and combine with puffed rice in a large mixing bowl and set aside.
In a medium saucepan, melt butter and add marshmallows. Stir continuously as the marshmallows start to melt. Once mostly melted, add vanilla and stir until you have a smooth mixture. Pour the mixture (careful, it'll be hot) over the pretzel/rice cereal mix and quickly stir it all together to fully coat. Pour the mixture into your lined pan and using a spoon that you've sprayed with cooking spray (to prevent sticking), pat the mixture down until its even and firmly packed.
Let cool for 3-4 hours, slice, and enjoy! Store leftovers in an airtight container.