Preheat oven to 375 degrees. Butter a 13 x 9 baking dish, set aside. In a mixing bowl whisk together all-purpose flour, corn flour, baking powder and salt, set aside. In the bowl of an electric mixer, whip together butter and sugar until light and fluffy about 3 - 4 minutes. Stir in egg, egg yolk, sour cream and lemon juice. Slowly mix in dry ingredients and stir until well blended. Gently press mixture evenly into buttered dish and bake 16 - 18 minutes until edges are lightly golden. Allow to cool completely then frost with Honey Butter Frosting. Store cookies in a single layer in an airtight container.
Honey Butter Frosting
Combine all ingredients in a mixing bowl and whip with an electric mixer until light and fluffy.
Notes
*Cornflour can be found in the health foods or gluten free section of the supermarket, if you can't find it there it is also sold online (here). I used Bob's Red Mill brand. This is entirely different from cornstarch so do NOT use cornstarch, I know some people refer to cornstarch as cornflour so I just wanted to clarify here that's not what you use in this recipe.