Preheat your oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan.
In a large bowl, whisk together the flour, granulated sugar, baking powder, baking soda, salt, and cinnamon.
In another bowl, cream the softened butter until light and fluffy. Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.
Gradually add the dry ingredients to the butter mixture, alternating with the buttermilk, starting and ending with the dry ingredients. Mix until just combined.
In a separate bowl, toss the diced apples with lemon juice and chopped nuts (if using). Fold the apple mixture into the batter gently.
Pour the batter into the prepared baking pan, spreading it evenly. Sprinkle the top with brown sugar.
Bake for 45-50 minutes, or until a toothpick inserted into the center comes out clean.
Allow the cake to cool in the pan for about 15 minutes before transferring it to a wire rack to cool completely.