Prepare the streusel swirl: In a small bowl, mix brown sugar, cinnamon, and flour together until evenly combined. Slowly pour in the melted butter and stir until the mixture becomes crumbly and slightly sticky. Set aside so it firms up slightly while you prepare the batter.

Make the pancake batter: In a large mixing bowl, whisk together flour, sugar, baking powder, baking soda, and salt. In a separate bowl, combine buttermilk, egg, melted butter, and vanilla extract. Pour the wet ingredients into the dry ingredients and gently stir until just combined. Be careful not to overmix—the batter should remain slightly lumpy for soft, fluffy pancakes.

Heat the pan: Preheat a non-stick skillet or griddle over medium heat and lightly grease it with butter or oil. Allow it to heat properly so the pancakes cook evenly and develop a golden-brown surface.

Cook the pancakes: Pour about ¼ cup of batter onto the hot skillet for each pancake. Immediately sprinkle a spoonful of the cinnamon streusel mixture on top of each pancake, gently pressing it into the batter. Cook for 2–3 minutes, or until bubbles form on the surface and the edges start to set. Flip carefully and cook for another 2–3 minutes until golden brown and cooked through. Repeat with the remaining batter.

Make the glaze: In a small bowl, whisk powdered sugar, milk, and vanilla extract until smooth and pourable. Adjust consistency by adding more milk if needed for a thinner drizzle or more sugar for a thicker glaze.

Assemble and serve: Stack the warm pancakes on a plate, drizzle generously with the vanilla glaze, and optionally add a little extra streusel on top. Serve immediately while warm so the cinnamon filling stays soft and slightly gooey inside.