01 - Preheat the oven to 350°F. Line two baking sheets with parchment paper.
02 - In a large bowl, whisk together flour, baking soda, salt, ground ginger, cinnamon, cloves, and nutmeg until evenly mixed.
03 - In a separate bowl, beat butter and dark brown sugar until light and fluffy. Add molasses and egg; mix until smooth.
04 - Gradually incorporate dry ingredients into the wet mixture, stirring until a cohesive dough forms. Divide dough in half, shape each into a disc, wrap in plastic, and refrigerate for 1 hour.
05 - Roll chilled dough to approximately 1/4 inch thickness. Using gingerbread house templates, cut out panels for front, back, sides, and roof. Arrange pieces on prepared sheets.
06 - Bake for 12 to 15 minutes until edges are firm but not browned. Allow panels to cool completely on racks before handling.
07 - Beat egg whites until foamy. Gradually add powdered sugar and lemon juice while continuing to beat until stiff peaks form. Transfer to piping bags for assembly and decoration.
08 - Pipe royal icing along edges of panels to glue them together, holding each piece gently until set. Attach roof panels last to complete the structure.
09 - Use remaining royal icing to affix assorted gumdrops, mini candy canes, chocolate buttons, sprinkles, and mini marshmallows to the assembled house. Allow icing to dry completely before display or serving.