Preheat oven to 350 F. Grease and line an 8-inch square cake pan with parchment paper. Don’t skip the parchment paper or you will be stuck wondering why half of your very moist cake is still stuck in the pan (this seemed to happen to our mother with alarming frequency, so we know from experience).
Place flour, salt, and almonds in the food processor and process until fine (1-2 minutes).Slice butter and chocolate into small pieces. Add chocolate and sugar and process briefly (30 seconds).Next, add the butter and continue processing until the batter forms a ball (1-2 minutes).Add eggs, one at a time. Process again. Add Grand Marnier, vanilla, and orange zest (if using).
Use a rubber spatula to scrape the the cake batter into the prepared pan.
Smooth the top of the cake and bake in the middle level of the oven 25-30 minutes, or until the cake begins to pull away from the pan.
Remove cake from the oven and place on a wire rack. Let the cake cool for 10 minutes and then remove cake from pan.