Initial Topping Layer: Immediately upon removing the crust from the oven, distribute ¼ cup of toffee bits evenly over its surface. In a separate vessel, combine ⅓ cup of sweetened condensed milk with 1 tablespoon of melted unsalted butter. Pour this mixture over the toffee bits, ensuring even coverage.
Additional Baking: Return the baking dish to the oven and continue baking for an additional 7 to 10 minutes. The criterion for completion is a visible bubbling of the mixture.
Final Topping Layer: After baking, place the dish on a wire rack for cooling. Subsequently, sprinkle 1 cup of milk chocolate chips over the top. Allow a brief period of 3 to 5 minutes for the chocolate chips to soften. Then, gently spread the softened chocolate using a small offset spatula, taking care not to disturb the underlying toffee layer. Immediately following, sprinkle the remaining ¾ cup of toffee bits over the chocolate, adding a sprinkling of flaky sea salt if desired.
Cooling and Setting: Allow the bars to cool at room temperature until they reach a state of equilibrium. If necessary, the bars may be transferred to a refrigerator to expedite the firming of the chocolate layer.