Weigh or measure the flour, salt, and baking powder, running a whisk through to blend and setting the bowl aside. In another bowl weigh or measure the sugar and set it aside. Zest and juice one lemon into separate containers and set these aside. I can usually get 3 tablespoon of juice from one lemon.
In a mixer bowl slice the butter. On a medium mixer speed, cream the butter until it has soft peak. Usually, a minute or two. Add sugar and mix for 3 minutes.
Add one egg at a time, mixing thoroughly before adding the 2nd egg. I stop the mixer and scrape everything down before adding the lemon zest, lemon juice, vanilla extract, and the mascarpone cheese. Blend for about a minute. Add the blueberries and mix for about 30 seconds. You want to mix until the blueberries are incorporated into the cookie dough. Shut the mixer off and take the bowl off the stand. Get a good spatula and fold the flour mixture into the cookie batter bowl. Once mixed, cover and refrigerate for at least 2 hours.
Preheat the oven to 350°F (175°C or 180°C), line 2 cookie sheets with parchment paper, and set aside. Take the cookie dough out of the refrigerator and add 12 scoops to a cookie sheet pan using a medium cookie scoop. Bake for 12-14 minutes, and they are just starting to get a little brown around the bottom edge of the cookie. Pull the cookie pan out of the oven and leave the cookies on the pan for a minute. Transfer to a cookie cooling rack. After they cooled, I sprinkled powdered sugar on top of the cookies.