Set the oven to 350°F, then fit aluminum foil into an 8x8-inch pan and coat it with cooking spray. Leave foil extending above opposite sides so the finished slab has handles for lifting.

Microwave the butter in a large microwave-safe bowl on high for about 90 seconds, until melted. The butter should be liquid with no solid pieces remaining.

Whisk the brown sugar and vanilla into the melted butter. The mixture should look evenly blended, without pockets of dry sugar.

Stir in the flour, rolled oats, baking soda, and a pinch of the listed salt if desired. Expect a thick, coarse mixture with visible oat flakes rather than a pourable batter.

Press roughly half of the oat mixture into the lined pan with a spatula or the back of a spoon, reserving the rest. A level layer reaching all four corners will support the filling evenly.

Bake the bottom crust for 10 minutes and prepare the caramel during this first bake. The surface should begin to set but does not need to be deeply browned.

Combine the caramel squares, heavy cream, and salt to taste in a large microwave-safe bowl, then microwave on high in 60-second intervals, stirring between intervals, for about 4 to 5 minutes total. Stop when stirring produces a smooth sauce without unmelted candy pieces.

Alternatively, melt the caramel squares and cream in a medium saucepan over medium-low heat, stirring almost continuously. The filling is ready when it becomes uniform and flows from the spoon in a thick ribbon.

Take the crust out after its 10-minute bake and distribute the chocolate chips or chunks across it. Even coverage gives each square a chocolate layer rather than isolated pockets.

Pour the warm caramel gradually over the chocolate, moving across the pan as you pour. The sauce should cover the chocolate without collecting entirely in one spot.

Break the reserved oat mixture into crumbs and scatter them over the filling. Loose clusters with small gaps allow caramel to peek through the topping.

Bake the assembled bars for another 15 to 18 minutes. Look for lightly browned edges and gentle bubbling near the center.

Leave the bars in their pan until completely cool, allowing up to 4 hours at room temperature, or use the refrigerator to help them firm with foil covering the top. The center should hold its shape instead of flowing when you begin cutting.