Step 1: In a large bowl, whisk together the flour, granulated sugar, baking powder, baking soda, and salt. Set aside.
Step 2: In a separate medium bowl, whisk together the buttermilk, egg, melted butter, and vanilla extract until well combined.
Step 3: Pour the wet ingredients into the dry ingredients. Stir gently with a whisk or spatula until just combined. Do not overmix; a few lumps are perfectly normal. Gently fold in 1 cup of crushed Oreo cookies.
Step 4: Heat a lightly buttered griddle or non-stick pan over medium heat. Pour about ¼ cup of batter for each pancake onto the hot griddle. Cook for 2-3 minutes per side, or until golden brown and cooked through. Flip once bubbles appear on the surface and the edges look set.
Step 5: While pancakes are cooking, prepare the glaze. In a small bowl, whisk together the powdered sugar, milk (or heavy cream), and vanilla extract until smooth. Add more milk, a teaspoon at a time, if a thinner glaze is desired.
Step 6: Serve the warm Oreo pancakes immediately, drizzled generously with the vanilla glaze and topped with extra crushed Oreo cookies.