Grease a baking tray (10 x 13) with butter.
Measure the peanuts in a bowl and keep it ready. Also measure the butter and vanilla in a small bowl and the baking soda in another small bowl and keep it ready near the cooking surface. We will be using it really quickly once the sugar syrup reaches the right consistency and hence it is important that we have everything measured and ready to add.
In a heavy bottom pan, combine the sugar, water and the corn syrup.
Heat it in medium flame until the sugar melts and the mixture starts to boil.
Keep boiling the sugar until it starts to thicken.
Once the syrup reaches a soft ball stage, add the peanuts. I made the brittle without the candy thermometer. Soft ball stage is when you drop some syrup into a bowl of cold water and you can gather the syrup as ball that is soft and squishy. The syrup should be able to hold its shape and not melt when dropped in water. If using a candy thermometer, it will be around 235 F.
Reduce the heat to medium low and keep stirring the mixture until it starts to thicken further and the syrup changes to light golden color. If using a candy thermometer, it should reach a hard crack stage (about 305 to 310 F).
Turn off the flame and quickly add the butter, vanilla and baking soda. Stir well for the butter to melt and you will notice that the mixture will start to foam up.
Once the mixtures foams up evenly and the butter has melted, quickly pour it onto the prepared baking sheet and spread as thin as possible. Use a fork to separate and spread if the mixture is too thick. All this has to be done very quickly.
Let the brittle cool down completely (at least an hour) and then break it into pieces and enjoy. Store them in a airtight container after they have cooled down completely.