Melt butter in a large saucepan over low heat. Add marshmallows and stir constantly until completely melted. Remove from heat. Add rice krispie cereal and crushed cookies, stir until well coated. Using a buttered spatula or waxed paper, press mixture evenly into a buttered 13 x 9 pan.

In a large mixing bowl, beat egg whites with an electric mixer on medium-low until foamy, 1 to 2 minutes. Add pinch of salt, cream of tartar, and vanilla extract on high for 2 to 3 minutes. With mixer on high gradually add sugar, one teaspoon at a time. Continue to beat on high until egg whites form stiff peaks.

Carefully spread egg whites to edges of baking dish over marshmallow treats. Place baking dish 3 inches below broiler element until egg whites are golden brown, watching carefully.