Preheat oven to 350 degrees F (180 degrees C). Coat a deep 9-inch cake pan or 9-inch springform pan with cooking spray.
Combine Butter (6 tablespoon) and Brown Sugar (1 cup) in a mixing bowl. Beat with a mixer until fluffy. Beat in Egg (2), one at a time, until well blended.
Combine Graham Cracker Crumbs (3/4 cup), Digestive Biscuit (3/4 cup), Whole Wheat Flour (1/2 cup),Baking Powder (1 teaspoon), Baking Soda (1/2 teaspoon) and Salt (1/2 teaspoon). Add crumb mixture to butter mixture, alternately with Buttermilk (1 cup), beating until well blended.
Scrape batter into prepared pan. Lightly tap against the counter to release air. Sprinkle top with Raw Cane Sugar (1/2 teaspoon) and Sea Salt (1/2 teaspoon), if using.
Bake 40 minutes, or until a wooden pick inserted in center comes out clean.
Let cool on a wire rack 10 minutes. Remove from pan and let cool completely. Serve and enjoy!