Preheat the oven to 350 degrees F. Lightly grease a 8- or 9-inch square baking pan with cooking spray and set aside.
Combine the sliced strawberries with the sugar and vanilla. Cover and let refrigerate for at least 30 minutes (or up to several hours).
For the whipped cream, beat the cream, powdered sugar and vanilla together (a blender works great for this) until thick. Refrigerate until ready to use (don't refrigerate longer than an hour or so or it will get a bit liquidy).
For the shortcake, in a medium bowl, cream together the sugar and butter for 1-2 minutes with a handheld (or stand) electric mixer. Add the egg and vanilla and mix well.
In a separate smaller bowl, whisk together the flour, baking powder and salt (see the note above for the chocolate version).
Add 1/3 of the dry ingredients to the batter and mix. Pour in half the milk and mix again. Add another 1/3 of the dry ingredients and mix followed by the rest of the milk. Mix in the milk and add the remaining dry ingredients. Mix until just combined.
Spread the batter evenly in the prepared pan. Bake for 20 minutes, until a toothpick inserted in the middle comes out clean (don't overbake or it will be dry).
Let the shortcake cool. Cut into squares, slice in half horizontally (like a hamburger bun). Place the bottom half on a plate, top with a spoonful of strawberries and a dollop of whipped cream. Top with the other half of shortcake and top with additional berries and whipped cream. Serve immediately.
Notes
For the Chocolate Version: replace the 1 1/2 cups flour with: 1 1/4 cup flour + 1/4 cup cocoa powder + 1/2 teaspoon baking soda.Ingredient Amounts: also, the amount of strawberries (and cream for that matter) you use is really dependent on how loaded you want each serving of shortcake. I've given a general amount but increase if you'd like.