Preheat oven to 350 degrees.  Line an 8×8 baking dish with parchment paper or foil and set aside.

To prepare crust and topping, combine sugar, flour, salt, and baking powder in a medium sized mixing bowl and stir to combine.

Use a large-hole cheese grater to grate butter into flour mixture, or alternately cut butter into small chunks and add to flour mixture.  Use clean hands to break up butter and flour mixture until it resembles large crumbs.  Add egg yolk from separated egg (reserve egg white for the next step) and add to crumb mixture.  Toss to incorporate egg yolk into crumbs.

Remove half of crumb mixture (about 1 ½ cups) and lightly press into the bottom of prepared baking dish.  Bake for 10 minutes and then remove from oven.

Add lemon zest to the remaining crumb mixture, stir to combine, and set aside.

To prepare filling, place ricotta and sugar in a food processor and process until smooth and creamy (about 1-2 minutes), scraping down sides as necessary.  Place mixture in a mixing bowl and stir in flour, vanilla, eggs, plus the reserved egg white from the first step, and lemon juice.  Whisk to combine and pour mixture over crust in pan.

Toss 3/4 cup berries with 1 1/2 teaspoon flour and sprinkle over the top of filling and then top with the reserved crumb mixture.  Bake for 40-50 minutes.  Bars should be just golden on top and slightly jiggly in center when shaken.  Remove and let cool completely.  Cover, and Place in fridge overnight to chill (or at least 8 hours).  When ready to serve, slice into bars and serve with sauce if desired.

For the Sauce,  combine all ingredients: frozen blueberries, lemon juice, water, and sugar.  Bring to a simmer and cook for 3-5 minutes, smashing berries with a fork.  Cool.  Sauce will thicken once cooled and chilled, adjust consistency by adding water until desired consistency is reached.