In a large bowl, whisk together the buckwheat flour, gluten free flour, brown sugar, baking powder, baking soda, and salt until well combined. Make a well in the middle of dry ingredients.
In a separate small bowl, mash the banana with a fork then whisk in the milk, lemon juice, melted coconut oil, beaten egg, and vanilla extract until well combined.
Heat a large nonstick skillet over medium heat then coat with cooking spray. Add heaping 1/4 cups of batter to the skillet then cook, topping each pancake with 3-4 apple slices and a sprinkle of cinnamon and coarse sugar when bubbles start to form, until undersides are golden and edges are set (about 2-3 minutes). Flip pancakes then cook an additional 2-3 minutes more. Transfer cooked pancakes to oven to keep warm then repeat with remaining batter. Serve with butter slices and maple syrup then enjoy!