For the blueberry compote: Combine the blueberries, sugar, lemon zest and 2 tablespoons of the lemon juice in a medium saucepan over medium heat and cook until the sugar dissolves and the mixture is thick and syrupy, about 10 minutes. (Don't let the berries cook too long or they will become mushy and lose their beautiful shape.) Remove from the heat. Gently stir in the chopped mint if using. Keep warm.
For the hotcakes: Whisk the ricotta and the egg yolks together in a large bowl to combine, then whisk in the buttermilk. Sift the flour, sugar, baking powder and salt into the ricotta mixture. Stir with a whisk until just combined.
Using an electric mixer, beat the egg whites in a large bowl until stiff peaks form. Using a large rubber spatula, gently fold the egg whites through the batter in 2 batches. Gently fold the fresh blueberries into the batter.
Melt some butter on a hot griddle pan over medium-low heat. Ladle the batter onto the griddle (you should be able to fit 3 hotcakes at a time) and cook until the hotcakes puff, become golden brown and are just cooked through, about 3 minutes per side.
Transfer the hotcakes to plates. Spoon the warm blueberry compote over the hotcakes, and then top with a dollop of butter. Serve immediately.