1) To make the dough: In a large saucepan, heat the buttermilk and butter until the butter is just melted; remove from the heat.
2) Add the brown sugar and molasses, then beat in the egg.
3) Whisk the baking soda, spices, and salt with 1 cup of the flour.
4) Add this to the wet mixture and mix until incorporated.
5) Add flour 1 cup at a time until you have a smooth, stiff dough. It should be stiff enough to be flexible, and neither crumbly nor sticky.
6) Divide the dough in half, flatten each half, and wrap in plastic. Refrigerate for at least 1 hour.
7) Preheat the oven to 350°F.
8) Using a quarter of the dough at a time (return the unused portion to the refrigerator), roll the dough 1/4" thick on a piece of parchment.
9) Cut your construction pieces as needed, pulling away the scraps to be re-rolled. Transfer the dough, parchment and all, to a baking sheet.
10) Bake the dough for 15 to 18 minutes, until set and very lightly browned at the edges.
11) Remove from the oven and trim any rough edges while the pieces are still warm. Cool completely before using for construction.
12) To make the royal icing: In a large bowl, whip the egg whites with the cream of tartar until foamy.
13) Sprinkle in the sugar gradually, whipping all the while. The more you whip the icing, the stiffer it'll be and the faster it'll harden up.
14) Cover the bowl of icing, taking out only as much as you'll need immediately. The easiest way to store the icing for long periods of time is in a plastic pastry bag, or zip-top food storage bag.
Yield: enough dough for a 10" x 7" house (approx.), plus decorations.