Fit parchment into an 8-inch or 9-inch square baking dish, leaving edges extending above the sides, or choose a 9x13-inch dish for thinner bars. The paper should cover the bottom and provide handles for lifting the firm slab.

Heat the butter and brown sugar together in a large saucepan over medium heat, stirring until melted and dissolved, then incorporate the vanilla followed by the oats. The butter-sugar mixture should look glossy rather than gritty before the oats go in.

Reduce the heat to low and stir the oat mixture for 3 to 4 minutes, until thoroughly combined. Look for evenly coated flakes with no dry pockets.

Transfer half the warm oats to the lined dish and compact them into an even bottom layer, keeping the other half in the saucepan. The crust should reach the corners without loose patches or deep gaps.

Microwave the chocolate chips and peanut butter together in a small microwave-safe bowl, stirring as they melt. The finished filling should look smooth and uniform, with no unmelted chips.

Set aside about 1/4 cup of filling for the drizzle, then spread the rest across the oat crust. A level chocolate layer will give each bar a similar amount of filling.

Distribute the reserved oats over the filling, press lightly, and trail the reserved chocolate across the surface. The topping should settle against the filling without pushing it out toward the edges.

Chill the pan for 2 to 3 hours or overnight, then let the slab come to room temperature before cutting. The layers should hold their shape, and the chocolate should yield to the knife rather than crack.

Control the heating carefully rather than extending it to make the mixture extra thick. Too little cooking can leave soft layers, while excessive cooking can produce dry, crumbly oats.