In a food processor, whip mascarpone cheese, blueberries with liquid, lemon zest, lemon juice, powdered sugar, and salt together until smooth. Some blueberry speckles are good, but overall texture should be spreadable. Transfer to air-tight container. Store chilled in a refrigerator below 40 degrees.
Blueberry Whoopie Pies
Preheat oven to 350 degrees (with no fan if possible).
In a mixing bowl, whisk together the flour, baking powder, salt, and sugar. Set aside.
In a separate larger mixing bowl, whisk eggs, vanilla extract, and blueberry puree together until thoroughly mixed.
Fold dry ingredient mixture into wet ingredients using a rubber spatula. Mix until just combined—do not over mix.
On a silicone or greased parchment-lined, full-sized baking sheet, portion 2 oz scoops of batter in a circle shape, spaced approx. 2 inches apart. Bake for 12 minutes, rotating pans halfway through. Remove cakes from oven and let cool on the sheet for 10 minutes. Transfer cakes from baking sheet to a wire rack to completely cool (approx. 1 to 1-1/2 hours).
Assemble whoopie pies in the following order: 1 cooled whoopie cake half, 1/4 cup whipped blueberry mascarpone filling, spread evenly over flat side of the cake half, 1 cooled whoopie cake half, flat side touching the filling.
Serve cool/room temperature. (Do not serve cold.)
Recipe Tip: If using a convection oven, preheat oven to 325 degrees, with fan on LOW. It is important to not overbake these cakes in order to keep a blue cake color and prevent cake browning.