Blueberry pie cookie bars

First make the filling:
Put blueberries, sugar, lemon juice and water in a medium saucepan. Stir to combine and cook over medium heat until the sugar is dissolved, fruit tender and the mixture is simmering, about 10 minutes.
Combine cornstarch with 1 tablespoon water in a small bowl. Stir it until smooth, then add to the blueberries. Keep whisking and bring the mixture almost to a boil, cook for a few minutes until until the sauce thickens and coats the back of a spoon. Remove from heat and allow to cool at room temperature.
As the filling is cooling, preheat the oven to 350°F (175°C) and butter an 8-inch (20-cm) square pan. Line it with parchment paper so it's sticking out at the sides. Set aside.
Make the crust:
Mix butter and granulated sugar in a large bowl. Using a hand mixer, beat on medium speed until light and fluffy, about 5 minutes. Beat in vanilla and egg. Lastly, add the flour in 2 additions and beat until completely the dough is smooth without any patches of flour.
Take 2/3 of the dough and press it into the baking pan with your fingers. Make sure it’s spread out evenly. Top with blueberry filling. Take the remaining dough and tear it up to pieces. Scatter those all over the blueberries. Sprinkle with flaked almonds and pearl sugar (if using).
Bake in the middle of the oven for 35-45 minutes or until the top is golden and lightly brown. Remove from oven and cool completely before slicing and serving. Store in the refrigerator for up to 3 days.