In a large bowl, whisk together the flour, sugar, baking powder, and salt.
In a medium bowl, whisk together the milk, ricotta, eggs, melted butter, lemon zest and juice, and vanilla.
Add the wet ingredients to the dry ingredients and mix until just combined. A few lumps are ok. If the batter is very thick, add milk, 1 tablespoon at a time, to loosen it slightly.
Heat a nonstick skillet over medium-low heat and brush lightly with butter or oil. Use a ⅓-cup scoop to pour the batter into the skillet. Cook for 1 to 3 minutes per side, or until the pancakes are puffed, golden brown, and cooked through in the middle. Reduce the heat to low as needed if the outsides of the pancakes are browning before the middles are cooked through.
Serve with maple syrup.
