In a large microwave safe bowl, add the butter, chocolate chips and marshmallows. Microwave for 90 seconds, then whisk until combined and smooth. Immediately pour in the cocoa Krispies and gently stir until coated.
Lay out a piece of parchment paper, approximately 18" long. Turn the coated cereal out on to the parchment and cover with a second sheet. Use your hands to press out the mixture, then use a rolling pin to roll out to approximately 16"x15." Freeze for 15 minutes.
Fit the template pieces on the cereal and use a pizza cutter or sharp knife to cut out the shapes. Leave the paper backing on the pieces.
To prepare the icing: Add the meringue powder, powdered sugar and water to the bowl of a stand mixer. Run on medium speed for 8-10 minutes, until soft peaks form. Spoon into a piping bag and cut the tip off.
Optional: Fit a second bag with a 1M tip. Place the bag with the prepared icing inside the bag with the tip, being sure that the frosting bag sits snugly down.
Use the frosting to glue the house together as in the video- first the sides, and then the ceiling. Support the walls with canned goods while the frosting sets.
Use remaining icing to attach desired decorations.