Preheat oven to 425°. Grease a sheet pan (a 12- by 17-in. rimmed baking sheet) with butter or cooking spray and line with parchment paper. Set aside.
Combine 2 cups blueberries, the lemon juice, and 2 tbsp. granulated sugar in a medium bowl. Let sit 15 minutes or so.
Combine flour, baking powder, and salt in another medium bowl. In a third medium bowl, beat egg whites with a mixer until soft peaks form.
In a large bowl, whisk egg yolks, ricotta, remaining 6 tbsp. granulated sugar, 6 tbsp. melted butter, the lemon zest, and milk to blend. Gradually add dry ingredients, whisking until smooth. Fold in egg whites and blueberry-sugar mixture. Spread in prepared pan, smoothing top with a spatula.
Bake until center is set and springs back when touched, 12 to 14 minutes. Remove pancake from oven and preheat broiler to high with a rack 5 in. from heat. Brush remaining 2 tbsp. butter over top of pancake. Broil until golden brown, 30 to 60 seconds.
Cut pancake into 12 squares and dust with powdered sugar. Serve with the remaining 1 cup blueberries and maple syrup.