For the Bars:
Preheat oven to 350?F. Line 9x9-inch baking pan with parchment paper.
In large heatproof bowl, combine butter and chocolate. Microwave in 20-30 second intervals, stirring after each, until fully melted and smooth. Whisk in sugar and cocoa powder, followed by eggs. Whisk in vanilla, salt and espresso powder. Stir in flour just until incorporated.
Pour batter into prepared pan. Bake, rotating halfway through baking time, until center is just set, about 20 minutes total. Transfer to wire rack to cool completely.
Once cool, spread marshmallow crème over top of bars. Evenly sprinkle with chopped pecans and toffee bits. Transfer pan to refrigerator or freezer to chill well.
For the Ganache:
In small saucepan, bring heavy cream to a brisk simmer of medium high heat. Remove from heat and add chocolate. Stir and let stand 1-2 minutes, then whisk together until smooth ganache forms. Add espresso powder and butter; whisk until smooth. Let mixture cool 30 minutes to thicken slightly. Drizzle ganache over top of bars; chill until ganache is fully set. Slice and refrigerate bars until ready to serve.