Melt chocolate gently for best results. If chocolate gets too hot, it may not set properly and will develop "bloom" (white streaks) on the surface when stored. Stirring chopped chocolate in a pan or bowl over hot, not simmering, water maintains an even, low temperature, resulting in glossy, firmly set chocolate. The setting time for the bark varies greatly according to the temperature of your home. Although we preferred the texture of the bark set at room temperature, you can set bark in the refrigerator if necessary.
2 pounds milk chocolate
1 tablespoon solid vegetable shortening
3 cups mini marshmallows, divided
3 cups coarsely chopped walnuts, divided
How to Make It
Line a 12- by 15-inch baking sheet with cooking parchment; butter parchment. Chop 2 pounds milk chocolate and place with 1 tablespoon solid vegetable shortening in a heatproof bowl that will nest in a 3- to 4-quart pan. Heat 1 inch of water in the pan just until steaming. Remove from heat and place bowl over water (bowl shouldn't touch water). Stir occasionally just until mixture is melted and smooth. Remove bowl from over pan.
Stir in 2 cups each mini marshmallows and coarsely chopped walnuts. Using a flexible spatula, scrape mixture onto buttered parchment and spread 3/8 to 1/2 inch thick (mixture should almost cover sheet). Sprinkle with 1 more cup each marshmallows and walnuts; gently press into chocolate.
Let stand at cool room temperature until completely firm, 4 to 6 hours, or overnight. Break or cut bark into pieces. Store airtight in a cool place up to 2 weeks.