
Combine the almonds, salt, and baking soda in a small bowl. Measure the butter and set aside next to the almond mixture.
Line a rimmed quarter sheet pan with parchment paper.
In a heavy-bottomed medium saucepan, combine the sugar and water. Stir to moisten the sugar, then set over medium heat and bring to a boil. Do not stir the mixture, but instead swirl the pan as it cooks.
When the sugar reaches a boil, add the butter and carefully set a candy thermometer in the pot. Reduce the heat to medium-low, swirling the pan every once in a while, until the mixture reaches between 310˚F and 350˚F, or a caramel color, known as the "hard crack" stage. Remove the pan from the heat and add the almond mixture. Carefully stir 4–5 times, or until the almonds are incorporated. Take care during this step, as the baking soda will cause the sugar mixture to bubble and expand.
Working quickly, transfer the mixture onto the prepared pan, spreading into an even layer with a rubber spatula.
Let it sit for about 30 minutes to harden, then break into 2-inch pieces.