Step 1Sift the flour, baking powder, sugar and salt into a large bowl.
Step 2In a medium bowl, whisk together the egg yolks, milk, melted butter and vanilla until combined. Whisk the wet ingredients into the dry; the result should be slightly lumpy yet combined to form a batter.
Step 3In another bowl, whip the egg whites to medium peaks, being careful not to overwhip. Using a large spatula, gently fold half of the egg whites into the batter, then fold in the remaining egg whites. The batter will be slightly lumpy with bits of egg whites not fully incorporated (it will look like white caps on the ocean with foam on top).
Step 4Heat a griddle over medium-high heat until hot (350 to 375 degrees if using an electric griddle). Grease the hot griddle with a little butter, then drop a quarter-cup of batter onto the griddle top. Add 2 tablespoons blueberries or sliced bananas and a scant tablespoon chopped walnuts before flipping the pancake. Flip the pancake when bubbles have started to form on top and the bottom is golden brown, 2 to 3 minutes, depending on the heat. Cook the other side until golden brown and the pancake is fully cooked, an additional 1 to 3 minutes.
Step 5Continue until all of the batter is cooked. This makes about 10 (3-inch) pancakes. Serve warm.