1. Make the crust: On a lightly floured work surface, roll pate brisee to a 13-inch round, about 1/8 inch thick. Fit into a 9-inch pie dish. Trim edge, and crimp as desired. Refrigerate 30 minutes.

2. Preheat oven to 375 degrees. Line crust with parchment and fill with pie weights or dried beans; bake until crust is set, 25 to 30 minutes. Remove pie weights and parchment, and continue to bake until bottom of crust is lightly browned, about 10 minutes. Remove from oven and let cool on a wire rack. Increase oven temperature to 425 degrees.

3. Make the bottom layer: Whisk to combine brown sugar and flour in a small bowl. In a medium bowl, whisk egg; add molasses and lemon zest and juice and whisk to combine. Add water, whisking until thoroughly combined. Set aside while you prepare the top layer.

4. Make the top layer: In the bowl of an electric mixer fitted with the paddle attachment, cream together the butter and sugar until light and fluffy, about 2 minutes. Add vanilla and egg; beat until well combined. Whisk together flour and baking soda. Add buttermilk in two additions, alternating with the flour mixture, beginning and ending with the flour. Scrape down sides as needed. Pour bottom layer into cooled crust, then dollop top layer over bottom layer to cover. Bake for 15 minutes. Reduce oven temperature to 350 degrees and bake until well browned, 35 to 40 minutes more. Transfer to a wire rack to cool completely. Serve with whipped cream, if desired.