Combine the baking soda and vanilla paste in a small bowl and reserve.
In a medium heavy-bottomed saucepan, combine the sugar, syrup, butter and 1 tablespoon water with a wooden spoon. Bring the mixture to 280 degrees F slowly over medium heat. Add the pistachios and cook, stirring constantly, until the mixture reaches 300 degrees F, or hard-crack. Remove from the heat and carefully add the baking soda and vanilla paste mixture, continuing to stir.
Carefully turn out onto the greased sheet tray and spread thinly and evenly with a spoon. Sprinkle sea salt flakes over the top. Let cool at room temperature until hardened, 20 to 30 minutes.
Break into pieces. Eat as-is or with vanilla ice cream. Store any remaining in an airtight container.