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1
Preheat oven to 350u00b0F with rack in middle. Butter a 13- by 9-inch baking pan. Line pan with foil, leaving an overhang at both ends, then butter foil.
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2
Whisk together flour, baking soda, spices, and salt.
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3
Beat butter and brown sugar with an electric mixer at medium speed until pale and fluffy, 3 to 5 minutes. Beat in egg until blended, then beat in molasses and buttermilk. At low speed, mix in flour mixture until smooth, then add hot water and beat 1 minute (batter may look curdled).
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4
Spread batter evenly in pan and bake until a wooden pick inserted into center comes out clean, 35 to 40 minutes. Cool in pan. Using foil as an aid, transfer gingerbread to a cutting board and cut into 1-inch cubes with a serrated knife.
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5
Sprinkle gelatin over cold water in a small saucepan and let soften 1 minute. Bring to a simmer, stirring until gelatin has dissolved. Whisk together gelatin mixture, pumpkin, brown sugar, spices, and salt in a large bowl until combined well.
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6
Beat cream with vanilla using cleaned beaters until it holds soft peaks, then fold into pumpkin mixture gently but thoroughly.
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7
Beat cream with sugar and vanilla using mixer until it holds soft peaks.
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8
Put half of gingerbread cubes in trifle bowl. Top with half of pumpkin mousse, then half of whipped cream. Repeat layering once more with all of remaining gingerbread, mousse, and cream. Chill at least 2 hours before serving.
