To make the filling, add the blueberries, sugar and lemon juice to a saucepan. Let simmer over medium-high heat for about 10 minutes.
Mix the cornstarch and water together in a small bowl and then stir into the blueberries. Remove from heat and let cool.
Take a layer of the phyllo dough and lay on a flat surface. Brush the top with a thin layer of butter. Place another sheet of phyllo dough on top of the first sheet and continue this pattern until you have a stack of four sheets.
Cut your stack of four sheets in half lengthwise as shown above. Place a heaping tablespoon of filling on the bottom and fold the corner up like a triangle.
Continue rolling the pastry up in a triangle motion until you reach the top of the dough. Repeat with the remaining ingredients which should yield about 8 pastries.
Once you have finished all the turnovers, brush the tops with more butter. Place on a baking sheet lined with a silpat and bake in the oven, preheated to 375 degrees F° for 15-20 minutes.
To make the icing, mix the confectioners sugar, vanilla and milk in the bowl of a stand mixer until thick and smooth, 2-3 minutes.
Transfer the icing to a piping bag fitted with a coupler and a Wilton size 5 tip. Frost the tops of each turnover once they have cooled. Enjoy!