Grease an 8-inch square pan and set aside.
Combine granulated sugar, brown sugar, butter, heavy cream, treacle, and cream of tartar in a medium saucepan. Cook over medium-high heat, stirring constantly until the butter is melted and the ingredients are combined.
Wash down sides of the pot with a pastry brush dipped in hot water if sugar crystals form on the sides.
Clip a candy thermometer to the pot and continue to cook without stirring until the mixture reaches 240°F on thermometer.
Remove pan from heat and stir in vanilla extract.
Allow bubbles to subside and the mixture to cool slightly (about 5 minutes). Remove the thermometer and stir vigorously with a wooden spoon until the mixture loses its gloss and is very thick (about 15–20 minutes).
Scrape mixture into greased pan. Use the palm of your gloved hand (or wrapped in plastic) to smooth top of the fudge.
Cool completely before cutting into 1-inch squares.