Combine sugar, corn syrup, and water in large saucepan over medium heat, stirring occasionally until the sugar is dissolved.
Put a candy thermometer in the pan, increase the heat to medium-high and cook without stirring until the mixture reaches 230 degrees F. Add the butter and toasted pecans, then stir the mixture constantly until it reaches 305 degrees F. When it reaches the desired temperature, it should be a deep caramel color and fragrant.
Working quickly, remove from heat and stir in baking soda, salt, and vanilla. The mixture will bubble up, so be careful. If the mixture hardens, keep stirring and put it over the still warm burner or low heat for a minute for it to come together.
Once the mixture is fully combined, pour the lined baking sheet. Butter the back of a wooden spoon and spread mixture as thin as possible. Work quickly; it will harden fast. Cool completely and then break into small pieces. Store in an airtight container for up to one month.