In a large bowl, mix the flour, baking powder and cornstarch.
Using a stand mixer, or an electric mixer, cream the butter and sugar until light and fluffy. Scrape the bottom and sides of the bowl using a rubber spatula.
Add the egg and vanilla extract and mix until fully incorporated. Scrape the bottom and sides of the bowl.
Reduce speed to low, then gradually mix in the flour. Pay attention not to over mix, the dough is ready when there is no visible dry flour.
Use your hands to bring the flour together over a clean work surface.
Wrap the dough with plastic wrap, and set it aside (or refrigerate) while you preheat the oven to 375F (190C).
Par-bake the dough
Grease the sides and bottom of an 8, or 9-inch square pan, and line it with parchment paper. For an easy release, allow the parchment paper to hang out of the pan.
Divde the dough into two parts: 2/3 and 1/3. Press the large part into the prepared pan, prick with a fork and bake for 18 minutes, or until the dough is puffy and the sides are light golden brown. Remove from the oven and chill for 15-20 minutes.
While the dough is baking, roll the remaining dough into 1/8 thick rectangle between two pieces of parchment paper and refrigerate.
Assemble and Bake
Spread the strawberry preserves over the par-baked crust.
Use a sharp knife to cut the chilled rolled dough into 1-inch stripes.
Gently and quickly lift each stripe and place them over the filling. Half horizontally and the other half vertically.
Bake at 350F (180F) for 18-22 minutes, or until the filling is bubbling and the topping is no longer shiny.
Remove from the oven and chill completely before inverting and slicing.
Notes
If the dough is too sticky, it could mean that the butter is starting to melt, refrigerate it for 30-60 minutes.
If the doug is too dry, you may have added too much flour or cron starch. Use a kitchen scale to measure the ingredients, use a fork to aerate the flour and spoon to scoop it into the measuring cup.
Make sure all of the ingredients are at room temperature before making the dough.
Store at room temperature for 2-3 days, or refrigerate for up to 5 days in an air-tight container.
To freeze: Wrap in plastic wrap, place in a freeze bag and freeze for up to 2 months. Unwrap to warm to room temperature.
For crumb topping: refrigerate the remaining dough, then use your fingers to crumble it over the filling. Bake.
Double the recipe and bake in a 9x13-inch if you wish to host a brunch or for a potluck.