Start with a flat board and cover it in foil (I used a chopping board).
Place a small container in the centre of the board to help hold up the chocolate walls.
Spread a little melted milk chocolate along the two long 'wall' sides of the board.
Place 1 full block on each side (using the container to help keep them standing up while the chocolate dries and sets the blocks in place).
Cut the remaining two blocks and secure with extra melted chocolate to form the front and back of the house (you'll have a few rows from each block leftover).
Use melted white chocolate to 'glue' on the front white chocolate door and the marshmallow decorations around the door (cut the marshmallows in half so they sit nice and flat).
To make the roof, cover two long rectangles with foil (I used plastic lids from containers but you could also use cardboard and cover it in foil too).
Lay the two rectangles flat on a bench and spread melted white chocolate over the entire area of foil. Lay the chocolate finger biscuits on the melted chocolate and place into the fridge to set.
Add a little melted chocolate to the top of the long walls and gently place the roof pieces in place (you might need to balance them a little bit!).
Add some melted chocolate to one side of the roof and 'stick on' the chimney.
Add melted chocolate to the top of the house and decorate with marshmallows.
Add marshmallows to the front of the roof (I actually didn't even need to use melted chocolate for this - I just squished them into place and they didn't move).
If you'd like extra chocolate Fingers around the edge of the house, simply spread over a little melted chocolate and stick them in place.
Make little snowmen with marshmallows and melted white chocolate (I used little edible silver baubles for the eyes).
Chop the Flake bars into small pieces and 'glue' them together with melted chocolate to resemble a stack of firewood.
Dust the entire board and house lightly with icing sugar to resemble snow.
Notes
This Chocolate Christmas House can be made ahead of time - simply cover loosely with foil and set aside until needed.