In a medium bowl, stir together the graham cracker crumbs and the melted butter until the mixture looks like wet sand. Press this firmly into the bottom of a parchment-lined 9x13 baking pan, using the back of a spoon to create a flat, even base.

In a separate small bowl, whisk the vanilla extract into the sweetened condensed milk. This adds an extra layer of aromatic sweetness to the binding layer.

Distribute the chocolate chips and the chopped walnuts evenly over the graham cracker crust. Pour the vanilla-infused milk over the top, ensuring it seeps into the crevices between the nuts and chocolate.

Slide the pan into a preheated 350°F oven. Bake for about 20 minutes. You want the edges to be bubbling and the center to be mostly set.

Pull the pan out and quickly scatter the miniature marshmallows over the surface. Return to the oven for an additional 3–5 minutes—just long enough for the marshmallows to puff up and turn a light, toasty brown.

Remove from the oven and immediately sprinkle the flaky sea salt over the warm marshmallows. This tiny addition transforms the bars from "sweet" to "sophisticated."

Allow the bars to cool completely in the pan before lifting out by the parchment paper and slicing into squares.