If your ricotta appears watery, place it in a fine-mesh strainer for about 30 minutes to drain excess liquid; if it looks dry and compact, skip this step.

In a small bowl, whisk together the flour, baking powder, sugar, and salt.

In a separate larger bowl, combine the ricotta, milk, egg yolks, and vanilla.

Add the dry ingredients to the ricotta mixture, stirring gently just until combined.

Using a handheld electric mixer (or a whisk by hand), beat the egg whites until stiff peaks form.

Stir one small scoop of the beaten whites into the batter to lighten it, then gently fold in the remaining whites with a spatula.

Heat a griddle over medium-high heat and melt just enough butter to lightly coat the surface.

Pour the batter onto the hot griddle using a 1/3-cup measure. Cook for about 3 to 4 minutes, until the undersides are golden and a few bubbles pop through the surface.

Flip and cook for another 2 to 3 minutes, until golden brown.

Repeat with the remaining batter.

Line a fine-mesh strainer with a paper towel, set it over a bowl, and add the ricotta; lightly press the surface with more paper towels to absorb as much moisture as possible, then let it drain for about 20 minutes.

In a food processor fitted with the steel blade, process the drained ricotta, sugar, vanilla, lemon zest, and lemon juice until very smooth, about 5 minutes, stopping once or twice to scrape down the sides.

Use the frosting immediately, or if it seems loose or overly wet, scrape it into a fine-mesh strainer lined with damp cheesecloth or a clean, damp dish towel, place the strainer over a bowl, cover, and refrigerate for several hours to drain further.