Stir together the chopped strawberries with the sugar, lemon zest, vanilla and cornstarch.
Crumb Layers
Preheat oven to 350 F. Line the bottom of an 8" round pan with parchment paper. If using an 8 inch square pan, line the bottom and sides and leave an overhang of parchment on two sides. Set pan aside.
In a large mixing bowl, whisk to combine the brown sugar, egg and vanilla extract. Set aside.
Mixing by hand option: In a medium sized mixing bowl, whisk to combine the flour and baking powder. Cut the cold butter into the flour with a pastry cutter until mixture forms crumbs roughly the size of peas. You can also do this using two forks. Food processor option: (You can also pulse the dry ingredients together in a food processor then add the chopped butter and pulse for about 20 seconds until pea sized crumbs form.)
Pour the flour/butter mixture into the sugar and egg mixture. Stir together with a spatula until combined.
Spoon about 2/3 of this mixture into the bottom of the baking pan. Use your hands to firmly press the mixture down so that it is compact. Spoon the strawberries evenly over the base. If there is any extra liquid from the berries in the bowl, discard the liquid.
Top the strawberries with the reserved 1/3 of the crumble mixture. It might clump together. Break the dough into little balls and scatter them throughout the top.
Bake at 350 F for 25-35 minutes or until the crust is lightly golden.
Remove from oven and place the pan on a cooling rack. Allow to cool completely.
Lemon Icing (optional)
Once cooled, stir the lemon juice and sifted powdered sugar together and drizzle over the entire crumble or over individual slices.
Notes
Strawberries: If you don't want to roast the strawberries, just chop strawberries and continue to stir them with vanilla extract and cornstarch. Use fresh or frozen strawberries. For frozen strawberries, thaw completely before using. Skip the roasting step if using frozen berries. Lemon Icing: Allow the bars to cool completely before icing. Instead of lemon icing, you can dust the cooled bars with powdered sugar or drizzle with melted white chocolate. Storage:Store leftover lemon strawberry crumble bars in an airtight container at room temperature for up to 3 days or refrigerate for up to 5 days. Wrap cooled strawberry crumble bars in plastic wrap and store in an airtight container for up to 2 months.