Place 12 cups popped popcorn in a large bowl or roasting pan.
In a medium saucepan, melt 6 tablespoons butter and 6 cups miniature marshmallows over medium-low heat, stirring constantly, until smooth.
Remove from heat and stir in 1 tablespoon vanilla, 2 tablespoons milk and 2 cups white cake mix Stir until cake mix is dissolved.
Pour the marshmallow mixture over the popcorn and stir to coat evenly. Top with sprinkles and spread onto a large, parchment lined baking sheet to cool.