01 - Combine mixed berries, maple syrup, and lemon juice in a small saucepan over medium heat. Bring to a gentle simmer, stirring occasionally. Stir in cornstarch slurry and cook for 1 to 2 minutes until slightly thickened. Remove from heat and keep warm.
02 - In a large mixing bowl, whisk together flour, sugar, baking powder, baking soda, and salt. In a separate bowl, combine buttermilk, eggs, melted butter, and vanilla extract until smooth. Add the wet ingredients to the dry ingredients and stir until just combined. Lumps are acceptable; do not overmix.
03 - Heat a nonstick skillet or griddle over medium heat and lightly grease with butter or oil. Pour 1/4 cup batter for each pancake onto the hot surface. Cook until bubbles form on the surface and edges begin to set, about 2 to 3 minutes. Flip and cook for an additional 1 to 2 minutes until golden. Transfer to a warm plate and cover loosely to retain heat. Repeat with remaining batter.
04 - Stack three to four pancakes per serving plate. Spoon warm maple berry compote over each stack, top with a pat of softened butter, and drizzle with warm maple syrup. Garnish with fresh mint leaves if desired.