Line a rimmed baking sheet with parchment paper. Measure out all of your ingredients and have them ready next to the stovetop. You will have to move quickly once the candy reaches 300°F.
Melt butter in a large saucepan over medium-high heat.
After the butter melts, stir in the sugar, light corn syrup, and water. Stir continuously until the sugar melts, about 2 minutes.
Place the candy thermometer in the sugar mixture and cook to 300°F, stirring frequently (about 15 minutes).
When the mixture reaches 300°F, stir in the baking soda.
Add the almonds and salted peanuts and quickly but carefully pour the mixture onto the prepared baking sheet. Using a knife, back of a large spoon, or a spatula, spread the brittle mixture out as evenly as possible.
Let the brittle cool fully (about 30 minutes) before breaking into pieces and enjoying.
