Melt butter in a pot set over medium low heat and stir in marshmallows and vanilla. Keep stiring until marshmallows have melted and combined with the butter.
Add Rice Krispies to a large bowl and pour the melted marshmallows into the bowl and stir to partially combine. Then let sit for seven minutes to allow marshmallows to cool.
Once cooled add the mini chocolate chips and stir to fully combine.
Place the mixture in greased a 9x13" pan and press flat. Cut into squares and enjoy.
Notes
If you don't let the marshmallows cool they will melt the chocolate chips.