Grease a baking sheet with butter or cooking spray and set aside. Lightly grease an offset spatula and set aside. You can line the pan with parchment paper, but if you do, make sure to grease the top of the parchment paper so the brittle doesn't stick.
Combine sugar, honey and water in a large pot fitted with a candy thermometer over medium heat, stirring to dissolve the sugar. Heat the mixture gradually until the temperature reaches 275˚F/135˚C. Don't rush this process – if it heats too quickly, too much moisture will evaporate and it will burn. Low and slow is best!
Stir in peanuts and pretzels until well coated, then continue heating over medium until the mixture reaches 300˚F/150˚C.
Remove from heat and immediately add in the butter, baking soda, and vanilla, mixing to combine. Be careful – the mixture will bubble up and change consistency.
Immediately pour the hot mixture onto the prepared cookie sheet and spread to the edges with the greased offset spatula (or a silicone spatula). Sprinkle with flaky salt (optional).
Once it's fully cooled, break the brittle into bite-sized pieces and enjoy! If you're having trouble getting the brittle out of the pan, gently bend the pan to pop it out.
