Generously grease a 9x9" baking pan with softened butter or neutrally flavored oil (like canola). Set aside.
Brown 4 Tablespoons unsalted butter in a small skillet over medium heat. Pour into a large, heat-proof bowl. Allow the butte to cool slightly and stir in 1/2 teaspoon vanilla extract and a pinch salt.
Line a baking sheet with parchment paper or silicone baking mat. Adjust oven rack to top position. Spread 10 ounces mini marshmallows evenly over baking sheet. Roast marshmallows under broiler, watching carefully. Remove from oven when golden brown.
Working quickly, scrape marshmallows into bowl, mix with butter and vanilla, then add 6 cups Rice Krispies and mix well.
Lightly press into prepared baking pan. Cover with plastic wrap and allow to set.