Prepare a cookie sheet with a silpat mat or parchment paper. If you are adding any ingredients to the toffee, sprinkle them onto the mat so that it is ready to go when the toffee is ready to be poured.
In a saucepan, place pats of butter, water, granulated sugar, brown sugar.
Place at medium heat and stir to melt.
When all ingredients are melted, place a candy thermometer into the batch. You are looking for the temperature to reach 300 degrees Fahrenheit or 150 degrees Celsius.
It will be bubbly and you'll stir every now and then. Once it hits temperature remove from heat.
Remove the thermometer. Stir in the baking soda and pure vanilla extract.
Once these ingredients have been mixed well, the texture will change to smooth with lots of air developed inside from the baking soda.
Pour it all onto the prepared cookie sheet.
Smooth it out. If you prefer thick toffee, do not smooth. I do this so that it's easier to bite and better to handle on it's own and inside other desserts.
Once you have smoothed out the batch, it should take up most of the cookie sheet. Allow it to cool completely before handling it. Once it has cooled/dried/set, you can break up the candy with your hands.