Put the butter, sugar, and extract, if using, in the bowl of a food processor or stand mixer. Sift in the strawberry powder to avoid lumps. Cream well. Make sure your strawberry powder is well incorporated and the mixture is streak free.
Add the flour and pulse about 10 times, then run the machine briefly, just until the dough comes together into a lump or lumps. If using a stand mixer, add the flour on low and mix until the dough comes together.
Turn the dough out onto a floured surface and finish bringing it together, if necessary, with your hands. When it is smooth with no crumbly spots remaining, form it into a disk. Wrap in plastic and refrigerate until firm, about 2 hours.
Preheat oven to 350F
Roll out the dough on a lightly floured surface to a 1/4 inch thickness and cut out with a cookie cutter. I used a 2 inch cookie cutter for about 20 cookies. I like to refrigerate the cut cookies for 15-20 minutes before baking.
Bake the cookies for 8 to 10 minutes, depending on the thickness of your cookies. I like my shortbread on the softer side, but if you prefer it crunchy, bake a little longer. Don't let them get browned.
Cool on the pan for about 15 minutes, until the cookies are firm enough to handle, and then lightly brush the edges with egg white, and roll in sparkling sugar. Let dry before eating. Note: you can also brush the edges and roll in sugar before baking, if you prefer.
How to make strawberry powder from freeze dried berries: