3/4 cup butter, melted (1 1/2 stick; I use unsalted butter but salted may be used)
3/ 4 cup light brown sugar, packed
1 tablespoon vanilla extract
1 cup all-purpose flour
1 cup whole-rolled old fashioned oats (not instant or quick cook)
1 teaspoon baking soda
pinch salt, optional and to taste
35 caramel squares, unwrapped (I used Werther's Originals Baking Caramels which are slightly bigger than cellophane-wrapped caramels; if using those, I recommend using a couple more)
1/2 cup heavy cream
1/2 teaspoon salt, optional and to taste, if you prefer salted caramel sauce
1 cup (6 ounces) semi-sweet chocolate chips or chunks (I used a combo)
Preheat oven to 350F. Line an 8×8-inch pan with aluminum foil and spray with cooking spray. Lining your pan is highly recommended for ease of cleanup due to the stickiness of the caramel; set pan aside.
In a large microwave-safe mixing bowl, add the butter and heat on high power to melt, about 90 seconds.
Add the brown sugar, vanilla, and whisk until smooth.
Add the flour, oats, baking soda, optional pinch salt, and stir until combined. Mixture will be quite thick.
Add half of the mixture to the prepared pan (just eyeball it), and smooth it with spatula or the back of a spoon, to create an even, smooth, flat layer; set remainder aside.
Bake for 10 minutes. While it bakes, make the caramel sauce.
In a large microwave-safe mixing bowl, combine the caramels, cream, and heat on high power in 60-second bursts to melt caramels, stirring after each burst. It will likely take about 4 to 5 minutes total to melt; heat until mixture can be stirred smooth. Alternatively, combine caramels and cream in a medium saucepan, and heat over medium-low heat to melt, stirring nearly continuously, until mixture can be stirred smooth.
Optionally stir in 1/2 teaspoon salt, or to taste, for salted caramel sauce; set sauce aside.
After 10 minutes, remove pan from the oven and evenly sprinkle with the chocolate.
Slowly and evenly pour caramel sauce over the chocolate.
Evenly crumble reserved oatmeal-brown sugar mixture over the top.
Return pan to oven and bake for about 15 to 18 minutes (I baked 16 1/2), or until edges are lightly browned and center is bubbling slightly.
Allow bars to cool completely in the pan before slicing and serving, giving the molten caramel time to firm up. This can take up to 4 hours, or overnight, at room temperature. You can speed it up by placing pan in fridge with a sheet of foil over the top.