Preheat oven to 350 degrees. Spray an 8x8 baking pan with non-stick spray or line the pan with foil and spray the foil with non-stick spray.
In a small bowl, add the brown sugar and cinnamon. Stir to combine and set aside.
In a large bowl, add granulated sugar and the butter. Beat with a hand mixer until creamed.
Add the vanilla extract, one egg, buttermilk and beat until well combined. Scrape down the sides and bottom of the bowl.
Add the second egg and beat until the egg is incorporated. Set aside.
In a separate bowl, add the baking powder and flour. Stir to combine.
Add the dry ingredients to the wet ingredients. Fold in with a spatula until the flour is worked into the batter.
Fold the grated or diced apple into the batter.
Pour the batter into a prepared baking pan. Spread the batter out with a spatula.
Sprinkle the brown sugar/cinnamon mixture all over the top and swirl it through the batter using a skewer.
Top with the walnuts.
Place in the oven and bake for 40-50 minutes or until done. Insert a toothpick into the center of the cake and if it comes out clean or with crumbs, it's done.
As soon as the cake comes out of the oven, prepare the glaze to pour over the top.
Glaze
In a medium bowl, add in vanilla, 2 tablespoon of heavy cream, and powdered sugar. Mix with a spatula until combined.
Add an additional 1 tablespoon of heavy cream at a time to the powdered sugar until the glaze is pourable.
Drizzle all over the top of the warm cake and cut to serve.