I figured Martha has an entire army over there and I'm sure they have tried this recipe a million time to make sure it works Perfectly. So here is her recipe for Gingerbread houses.
1 teaspoon baking soda
1/2 teaspoon baking powder
1 cup (2 sticks) unsalted butter
1 cup dark-brown sugar
4 teaspoons ground ginger
4 teaspoons ground cinnamon
1 1/2 teaspoons ground cloves
1 teaspoon finely ground black pepper
1 1/2 teaspoons salt
2 large eggs
1 1/2 cups unsulfured molasses
6 cups all-purpose flour
In a large bowl, sift together flour, baking soda, and baking powder; set aside.
In an electric mixer fitted with the paddle attachment, cream butter and brown sugar until fluffy. Mix in spices and salt. Beat in eggs and molasses.
Add flour mixture; mix on low speed until thoroughly combined. Divide dough into thirds and wrap in plastic. Chill for at least 1 hour.
Heat oven to 350 degrees. On a well-floured surface, roll out dough to 1/8 inch thick. Cut into desired shapes, or using template instructions, if desired. Place dough shapes on ungreased baking sheets; chill until firm, about 15 minutes. Bake 15 minutes, or until gingerbread is firm in the center, but not dark around the edges.
In large bowl of stand mixer combine the egg whites and vanilla and beat until frothy. Add confectioners' sugar gradually and mix on low speed until sugar is incorporated and mixture is shiny. Turn speed up to high and beat until mixture forms stiff, glossy peaks. This should take approximately 5 to 7 minutes. Add food coloring, if desired. For immediate use, transfer icing to pastry bag or heavy duty storage bag and pipe as desired. If using storage bag, clip corner. Store in airtight container in refrigerator for up to 3 days.