Preheat your oven to 350 °F. Line a rimmed baking sheet with foil, covering the bottom and sides. Spray with nonstick cooking spray. Place a layer of matzo crackers on the sheet, breaking as needed to cover the sides; set aside.: Warmth from preheating ensures the caramel sets uniformly once it hits the matzo crackers , and lining the pan with foil makes removal effortless later. As the oven warms, listen for the quiet hum and feel the gentle rise in kitchen temperature, which helps the caramel behave predictably. A common mistake is skipping the foil, which can make cleanup and removal messy; also, if the crackers are packed too loosely they may shift when pouring the caramel, so press them gently to create an even base.
In a heavy 2-quart saucepan, melt the butter over medium heat. Stir in the sugar and 2 tablespoons of water. Bring the mixture to a boil and cook undisturbed over medium-high heat until the caramel mixture is a light golden and registers 255 degrees on a candy thermometer, about 5-7 minutes. Remove from the heat and let cool for 1 minute. Stir in the vanilla and quickly pour over the matzos, using a metal spatula to spread evenly. Bake for 8 minutes.: As the butter and granulated sugar combine, you will notice a glossy sheen and a toffee aroma blooming in the pan; those are signals the caramel is progressing. The sound will shift from gentle bubbling to a more vigorous boil as water evaporates. Hitting 255 degrees yields a firm but slightly tender caramel after baking. Cooling for a minute prevents the vanilla from evaporating and tempers the caramel so it will spread rather than running off the matzo crackers . A frequent error is stirring constantly early on which can cause crystallization, so patience is crucial.
Remove the bark from the oven and sprinkle with the chocolate chips. Return to the oven and bake just until the chocolate starts to melt, about 2 minutes, then spread the chocolate evenly.: When the sheet returns to the oven, watch the chocolate chips closely; you want them glossy and soft, not fully pooled. The brief warmth will transform the chips into a spreadable state, and when you run a spatula over them they should give with a silk like texture. The smell of warm chocolate will become more forward. Overheating here can make the chocolate grainy, so remove promptly when just softened and spread smoothly in thin strokes for a uniform layer.
Meanwhile, melt the peanut butter in a microwave safe bowl for 1-1 1/2 minutes, until it is a pourable consistency. Drizzle the peanut butter over the chocolate and, using the handle of a wooden spoon, swirl the chocolate and peanut butter together.: The melted peanut butter should be pourable, with a glossy sheen and a faint roasted aroma. Drizzling creates ribbons that you can manipulate into decorative swirls with the spoon handle, producing marbled visual appeal and pockets of concentrated flavor. If it is too thin, it will bleed into the chocolate; if too thick, it will sit in lumps, so heat in short bursts until it reaches the right flow. A common misstep is overheating peanut butter, which can make it oily and separate, so warm gradually.
Let the bark cool for 1 hour, then refrigerate until set, at least 2 hours. Break into pieces and serve.: As it cools, the chocolate will firm and the caramel will snap into place, offering a crisp bite followed by a chewy middle. You will notice the surface go from glossy to matte as it fully sets. Cooling on the counter allows residual warmth to dissipate gently before chilling, which helps avoid condensation. A usual pitfall is breaking the bark too soon, which yields sticky, soft pieces; patience results in clean breaks and tidy shards.