In a large bowl, gently whisk together the flour, sugar and baking powder and salt, make a well in the middle and add slightly beaten egg and yolk, vanilla and cubed butter. With a fork mix together until dough slightly comes together, then turn mixture onto a slightly floured surface and work gently to form a soft dough. Wrap in plastic and refrigerate for 30 minutes.
FOR THE FILLING
In a medium sauce pan add strawberries, sugar and cornstarch, cook over medium heat until thickened (I left a little juice) approximately 10 minutes, remove to a medium-sized bowl and let cool completely.
Meanwhile, Pre-heat oven to 340° (170° celsius), line cookie sheet with parchment paper, and lightly grease and flour 2 or 3 tart pans if using.
Remove dough from fridge, knead the dough a couple of times to soften it up on a lightly floured surface.
Roll out 1/2 the dough (easier to work with) at a time to 1/8 inch thickness and cut out with a round type of cookie cutter.
In the middle of each cookie add approximately 1 teaspoon of strawberry filling, top with second cookie and seal with fingers dipped in water if necessary.
Continue until all cut outs are complete (and 2 or 3 tarts if desired).
Bake for approximately 13-15 minutes, (cookies should not be golden, tarts approximately 5 minutes longer). Let cool slightly on cookie sheet and then move to a wire rack, dust with powdered sugar if desired before serving. Enjoy!
** Depending on size of cutter makes approximately 10 sandwich cookies, 3 tarts, I also made small cut out cookies with leftover dough.