Line an 8x8" or 9x9" square pan with parchment paper and coat with nonstick spray. Or just spray a pan. Set Aside.
In a 3 QT heavy saucepan over high heat, combine sugar, butter, and evaporated milk. Use a wooden spoon to slowly stir until butter melts, scraping sides of pan.
Bring to a full rolling boil, stirring constantly with a long wooden spoon. Once it starts boiling, set a timer for 5 minutes and turn the heat down to medium/low so that you don't burn your fingers off-- it should be boiling the full 5 minutes. Stir constantly.
When the timer goes off, remove from heat and add chocolate chips. Stir until all chips are melted and mixture is smooth. Add marshmallow creme and beat with a wooden spoon until well-blended. Add vanilla and mix well. Immediately pour into the prepared pan.
Let cool to room temperature then slice and serve. Store on the counter, covered.
Notes
Make sure to not cook the butter, sugar and milk longer than 5 minutes! And turn the temperature down if it is boiling too much. For my stove I like it a little below medium.