Place pecans in a single layer on a baking sheet. Bake at 350 for 8-10 minutes until toasted.
Microwave 1 cup butter and semisweet chocolate in a large microwave-safe glass bowl at HIGH 1 minute or until melted and smooth, stirring every 30 seconds.
Whisk sugar and next 5 ingredients into chocolate mixture. Pour batter into a greased 15x10x1 inch jelly roll pan.
Bake at 350 for 20 minutes. Remove from oven and sprinkle evenly with miniature marshmallows; bake 8-10 more minutes or until marshmallows are golden brown.
Drizzle warm cake with warm Chocolate Frosting (recipe follows), and sprinkle evenly with toasted pecans.
Chocolate Frosting: 1/2 Cup butter (try substituting coconut oil for a dairy free version), 1/3 Cup unsweetened cocoa, 1/3 Cup almond milk, 2 Cups (16 oz. box) powdered sugar, 1 t. vanilla extract.
Stir together first 3 ingredients in a medium saucepan over medium heat until butter is melted. Cook, stirring constantly, 2 minutes or until slightly thickened; remove from heat. Beat in powdered sugar and vanilla at medium-high speed with an electric mixer until smooth.