Line a rimmed baking sheet with parchment paper and lightly coat with nonstick cooking spray. Combine sugar, corn syrup, and ½ cup water in a medium saucepan over medium- high. Bring to a boil and cook, undisturbed, until sugar dissolves, about 3 minutes.
Continue to cook, swirling saucepan occasionally, until edges turn golden; swirl pan to incorporate golden sugar into the rest of the mixture. When mixture is evenly amber in color, reduce heat to low and add Peanut Butter & Cocoa PEBBLES™ Cereal, butter, vanilla, and kosher salt. Cook, stirring, until evenly incorporated. Add baking soda (mixture will bubble up) and stir until evenly combined.
Transfer brittle to prepared baking sheet and spread to about inch thick. Sprinkle with flaky salt, if desired, and set aside until completely cool. Break into pieces. Brittle will keep in an airtight container at room temperature for up to 2 weeks.