Preheat the oven to 375°F (~190°C). Line a 13x9 pan with foil and add the butter directly to the pan.
Once the oven is preheated, place the pan in the oven and cook for around five minutes, until the butter is melted. Keep an eye on it to make sure the butter doesn’t burn!
Remove the pan from the oven and add enough graham cracker crumbs to cover the bottom of the pan and absorb the butter. Pat until firm. You may see a few wet spots, but you want the butter absorbed with a mostly firm base.
Sprinkle chocolate chips evenly across the graham cracker base.
Sprinkle butterscotch chips evenly on top of the chocolate chips, filling in gaps where possible.
Drizzle the sweetened condensed milk evenly over the top, especially around the edges. It will even out during baking, so don’t stress about making it perfect.
Bake for 15-25 minutes, until bubbling and just starting to turn golden brown around the edges. Do not overbake, as the bars will firm up after baking.
Cool at room temperature, then refrigerate before slicing for best results.
Trim edges using a sharp knife, then cut into desired-size rectangles.
Store in an airtight container at room temperature or in the refrigerator. Bring to room temperature before serving.