Melt butter in a medium saucepan set over medium low heat along with marshmallows, vanilla, and salt. Gently stir to ensure even melting and to prevent scorching.
Once marshmallows have melted into a creamy consistency remove the saucepan from heat and pour the marshmallow mixture into a large bowl over the Fruity Pebbles cereal.
Using a silicone spatula, stir to combine the cereal mixture while the marshmallows are still warm and spreadable. Stir gently to avoid crushing the cereal as much as possible.
Pour the treats into a greased 11x7 baking dish, and press mixture flat to fill the dish. Be careful not to over press them or they will crush into bricks.*
Let cool, then cut and enjoy.
Notes
*Other sized baking dishes can also be used, the thickness of your treats will just become thinner or thicker.