Line a rimmed baking sheet with parchment or aluminum foil. Combine the sugar, brown sugar, milk, butter, vanilla, and pecans in a medium sauce pan, at least 4 quarts. Do not use a smaller pan as the syrup will bubble up during cooking.
Cook the syrup over medium-high heat, stirring occasionally. When it comes to a boil, start stirring constantly. Let it boil for about 3 minutes, until the syrup registers 238°f – 240°F on a candy thermometer. Remove the pan from heat immediately and keep stirring. Stir, stir, stir!
It will become creamy, cloudy, and start to thicken. When you feel it starting to get grainy, the pralines are ready. You can also hear it if you listen closely; the crystals will make a scraping noise against the side of the pan.
Drop spoonfuls of the praline syrup onto your waiting parchment. Work quickly, as the syrup starts to set as it gets cool. Let cool and harden for at least ten minutes. Will keep in an airtight container for several days, but they’re at their very best within the first 24 hours of making them!