Preheat the oven to 350°F (175°C). Grease and flour a 9x13-inch baking dish.
In a large mixing bowl, combine the flour, granulated sugar, baking powder, baking soda, and salt. Mix well.
Add the softened butter, egg, vanilla extract, buttermilk, lemon juice, and lemon zest to the dry ingredients. Beat with an electric mixer on medium speed for about 2 minutes until well combined.
Pour the batter into the prepared baking dish and spread it evenly. Bake for 25-30 minutes or until a toothpick inserted in the center comes out clean. Allow the cake to cool completely in the pan.
In a separate bowl, whip the heavy cream with the powdered sugar and lemon extract until stiff peaks form.
Once the cake is cooled, spread the whipped cream mixture evenly over the top. Chill the cake in the refrigerator for at least 1 hour before serving.
Cut into squares and serve chilled.