Combine the pecans, granulated sugar, brown sugar, cream, butter, corn syrup and vanilla in a large Dutch oven fitted with a candy thermometer. Bring to a boil over medium heat, stirring constantly with a wooden spoon. Cook until the candy thermometer registers 236 degrees F, 6 to 8 minutes.
Remove from the heat and continue stirring for 1 minute; the mixture will begin to thicken. Working quickly, carefully pour the praline mixture onto the prepared baking sheet and spread thin before it starts to set. Allow to cool and harden for at least 10 minutes.
Break into serving-size pieces. Store in an airtight container.