To make the cookies: line two baking sheets with parchment paper or a silicone baking mat and set aside. Preheat oven to 350 degrees.
In the bowl of a stand mixer with a paddle attachment (or with a hand mixer) cream together the butter and sugar for about 1 minute. Scrape down the edges of the bowl and add in the vanilla extract, egg and lemon zest and beat until well combined, scrape down the sides of the bowl.
In another bowl stir together the whole wheat flour, salt, cornstarch and baking powder.
Slowly add the dry ingredients into the butter/sugar mixture, and beat until dough is combined.
Using a tablespoon size scoop - or your hands - measure out tablespoon sized cookie dough balls and place them on the prepared cookie sheets. Gently press on the balls to flatten them slightly, and press a dent into the center of the cookies (this is nice so that the frosting and berries go into the dent and stay on the cookie). The cookies don't spread too much when baking, but I don't overcrowd them, I place 12 cookies on a cookie sheet.
Bake cookies for 12 minutes. You want them a little soft still.
Remove cookies from oven and allow to cool for 15 minutes before transfering them to a wire rack to completely cool before frosting and topping them with fruit.
While the cookies are cooling, make the frosting by combining the cream cheese, sour cream, powdered sugar and lemon zest and beating it with a mixer until smooth and creamy.
Prep/clean the fruit and set it aside.
To assemble the cookies, spread a little bit of the frosting over the top of a cookie, and then top with fruit. For each cookie I put a 1/2 a slice of kiwi, a piece of strawberry, two blueberries, 1 raspberry and one blackberry - but you could easily use whatever combination of fresh fruit you'd like. Repeat until all cookies are frosted and have fruit on them. I put the cookies in the refrigerator for two hours before serving, this helped the frosting and fruit 'set'. The cookies came to room temperature in a matter of minutes and were amazing with the fruit being a little cool when served