- Make the lemon sauce: In a small saucepan, whisk together sugar and cornstarch. Gradually whisk in boiling water. Bring to a boil over medium heat, then reduce heat to medium-low and simmer until sauce is thick and resembles syrup, about 5 minutes. Add butter, lemon zest and juice, vanilla, and salt, and stir until butter has melted. Cool to room temperature before pouring over gingerbread.
- Make the bourbon chantilly cream: In the chilled bowl of an electric mixer with chilled beaters, whip cream to soft peaks. Sprinkle in sugar and beat until blended, no more than 30 seconds. Add bourbon, beating until stiff peaks form. Do not overbeat.
- Make the gingerbread: Preheat oven to 350 degrees. Lightly coat a 9×13-inch baking pan with butter and dust with flour, tapping out excess. In a bowl, whisk together flour, baking powder, baking soda, salt, ginger, cinnamon, and cloves. In another bowl, combine molasses and coffee.
- In a stand mixer fitted with paddle attach- ment, beat melted butter, brown sugar, and eggs on medium speed until light. Beat in flour mixture in three batches, alternating with coffee-molasses mixture, beginning and ending with flour mixture. Scrape down sides of bowl, then beat for 30 seconds more.
- Pour batter into prepared pan and bake until a wooden pick inserted in center comes out clean, 30 to 45 minutes. Cool in pan on a wire rack, then cut into squares and serve warm with lemon sauce and cream.