Grease a 9x13 inch pan and one foil lined 9 x 9 inch pan with butter; set aside.
In a 4-quart saucepan, combine the sugar, corn syrup, cream, milk and salt. Cook over low until combined and the sugar is dissolved.
Turn heat up to medium and clip a candy thermometer to the side of the pot Cook the mixture, stirring constantly with a heatproof rubber spatula, until the mixture reaches soft ball stage (for here in Utah that is 230 degrees) .
Remove the pot from the heat and stir in the vanilla and root beer extracts.
Pour the mixture into the greased 9X13 pan.
Leave the pan undisturbed to cool at room temperature for 1 hour. (Do not stir during this step or the fudge can turn to sugar.)
When cool enough, pour the mixture into the bowl of your kitchen aid.Using the paddle attachment mix for approximately 3 minutes. It is done mixing when the fudge begins to lighten in color, lose a bit of it’s shine and thicken slightly.
Pour mixture into the 9 x9 foil lined baking pan and spread evenly with an offset spatula.
Pour mini marshmallows evenly oven the fudge and lightly press into the fudge.
Allow the fudge to set and crystallize for 1 hour or longer at room temperature. It will lighten in color as it cools.
Remove fudge from the pan using the foil and cut into desired pieces and serve.
The fudge should be stored tightly covered at room temperature or refrigerated tightly sealed for longer storage.