Line a 9-inch square baking pan with parchment, leaving 2-inches overhanging on opposite sides.

Melt the butter in a large pot over medium heat. Stir in the brown sugar and the vanilla. Add the oats, reduce the heat to medium-low, and cook for 3 to 4 minutes, stirring frequently.

In another pot, place the chocolate chips, peanut butter, and salt and allow to melt over low heat, stirring frequently to prevent scorching. The mixture is ready when the chocolate is completely melted, with no visible chocolate pieces.

Spoon ⅔ of the oat mixture into the prepared baking pan, covering the bottom of the pan evenly. Use a spatula to firmly and evenly press the mixture into the pan. The more firmly the oats are pressed, the more stable the crust will be.

Pour the chocolate-peanut butter mixture over the oat crust, smoothing it out with the back of a spoon.

Finally, sprinkle the remaining oat mixture over the chocolate, pressing it gently into the chocolate.

Transfer to the refrigerator, and chill for at least three hours or overnight. Allow to sit at room temperature for 5 to 10 minutes before cutting and serving. Leftovers are best kept refrigerated, then brought out again when needed.