Grandma Marlene kept a pan of these tucked in the fridge, just waiting for after-dinner snackers.

Once cooled, cover the glass casserole dish tightly with plastic wrap or a lid and refrigerate for at least 3 hours, or overnight. This is when the magic happens—the bars firm up into those classic icebox squares Grandma kept tucked away for after-dinner snackers.
When ready to serve, remove the pan from the fridge and let it sit on the counter for about 5–10 minutes to soften just slightly. Use a sharp knife to cut into 16 small squares (or more, if you want bite-sized snackers). Wipe the knife between cuts for the cleanest edges.
Store any leftovers covered in the same glass dish in the fridge for up to 5 days. They’re best enjoyed cold, one little square at a time, whenever someone wanders into the kitchen looking for a sweet bite.
Variations & Tips
To lean even more into the butter pecan flavor, brown the 3 tablespoons of butter for the topping before mixing it with the sweetened condensed milk; it adds a toasty, caramel note. For a slightly less sweet version, use a scant can of sweetened condensed milk (about 1 1/4 cups) and add a splash of heavy cream to loosen it. If you like a thicker, bakery-style bar, press the crust into an 8×11-inch glass dish instead of 9×13 and add 5–8 minutes to the bake time. Swap in half walnuts or almonds if you’re low on pecans, or use a mix for more texture. For a chocolate twist, sprinkle 1/2 cup mini chocolate chips over the hot bars right when they come out of the oven, then gently swirl them into the top without fully mixing for a marbled effect. To make these a little more weeknight-friendly, crush graham crackers in a zip-top bag with a rolling pin to cut down on dishes, and assemble the bars the night before; they slice even better on day two. If you need to transport them to a potluck or family dinner, keep them in the same glass casserole dish, chilled, and cut them on-site so they look neat and hold their shape.