Spray a 9" x 5" loaf pan with cooking spray and set aside. In a saucepan set over low-medium heat, melt butter. Add marshmallows and stir to coat evenly with the melted butter. Stir marshmallows occasionally as they begin to lose their shape and melt. Once marshmallows have just melted completely, stir in cocoa powder and espresso powder until just fully combined.
Remove saucepan from heat and stir in cereal. Once cereal is coated evenly by the marshmallow mixture, stir in 1/2 cup chocolate chips.
Press mixture into one half of the prepared loaf pan, pressing on the top to ensure treats are dense. Allow to cool for a few minutes before proceeding.
Once cool, invert the Rice Krispie block onto a plate. Melt remaining 1/4 cup chocolate chips and spread evenly over top and sides of Rice Krispie block. Garnish with chocolate sprinkles or whatever other decorations you'd like. Let chocolate cool completely before slicing and serving (may refrigerate Rice Krispie block to speed up the hardening process).