1. Lay the first sheet of pastry flat and brush with the egg wash. Place the second sheet of pastry directly on top of the first and cut the pastry into 16 equal squares or rectangles depending on the particular size of the pastry sheets.
2. Place the squares onto a parchment lined baking sheet. Brush the tops of the pastries with egg wash and sprinkle the tops with a thin layer of plain table sugar.
3. Chill the pastries for a half hour in the fridge before baking.

4. Bake in a 375 F degree oven for about 20 minutes until medium golden brown. Cool the pastries on a wire rack. When cool, split each of the pastries in half horizontally and  fill with a layer each of blueberry jam and lemon buttercream frosting.

To make the Lemon Buttercream Frosting

1. In the bowl of an electric mixer combine the icing sugar and butter until the mixture resembles a coarse meal.
2. Add the lemon juice and lemon zest. Beat together well until smooth, light and fluffy. Depending on the size of the lemon you use, you may not have to use all the juice or you may be a little short; just add a little milk to bring it to the proper consistency. If it is a little too soft, just add a little more icing sugar.