Line a 9×9-inch baking pan with parchment paper.
In a bowl, combine graham cracker crumbs, melted butter, condensed milk, and vanilla until evenly mixed.
Press mixture firmly into the prepared pan to form the crust.
Sprinkle white chocolate chips evenly across the surface.
Add mini marshmallows, festive sprinkles, and chopped pecans if using.
Cover and refrigerate for at least 2 hours or until fully set.
Lift from pan using parchment, slice into 12 bars, and serve chilled.
