Preheat oven to 350 degrees. Butter a 9-inch-round-by-2-inch-tall cake pan; line with parchment.
Sift together flour, baking soda, salt, 1 teaspoon ginger, and 1 teaspoon cinnamon in a large bowl, and set aside.
Beat 8 tablespoons butter in electric mixer on medium-high speed until lightened, 3 to 4 minutes. Scrape down sides of bowl with rubber spatula. Add 1/2 cup sugar, 1/4 cup at a time; scrape down sides after each addition. Beat until fluffy, 3 to 4 minutes more. Add eggs, 1 at a time, beating 1 minute after each addition. Slowly add molasses, beating on medium speed, about 10 seconds. Slowly pour in buttermilk; beat to combine.
On low speed, slowly add reserved flour mixture in 3 parts, beating to combine after each addition. Transfer to prepared pan. Bake until cake tester inserted into center comes out clean, 35 to 45 minutes. Let cool on wire rack. Gingerbread may be made 2 days ahead or kept frozen for 1 month.
Juice lemon; place juice and both halves in large bowl of cold water. One at a time, peel pears, quarter, core, and cut into 1- to 1 1/2-inch chunks; transfer to acidulated water.
Drain pears; transfer half to 12-inch skillet. Add 1/2 cup sugar, 2 tablespoons butter, 1/4 teaspoon ginger, and 1/4 teaspoon cinnamon. Place over high heat; saute, stirring, until sugar and butter melt and form caramel, 4 to 5 minutes. Reduce heat to medium high, cover, and continue cooking 10 minutes more, stirring occasionally. Transfer pears and liquid to large bowl to cool; repeat with remaining pears, 1/2 cup sugar, 2 tablespoons butter, 1/4 teaspoon ginger, and 1/4 teaspoon cinnamon. Let cool completely.
Slice gingerbread into 3 rounds of equal thickness. Cut each round into 8 triangles. Line bottom of trifle bowl with 8 triangles of gingerbread; trim to fit if needed. Spoon 1 cup Cognac Custard over gingerbread, followed by 2 cups pears. Repeat process in two groups, layering with remaining gingerbread, custard, and pears.