Preheat oven at 350⁰F/177⁰C. Grease, line with non-stick paper a 9-inch square pan. Set aside
In a bowl, using a pastry blender or a fork, combine the flour butter, confectioner’s sugar and white sugar until well blended. Reserve 2/3 cup of the mixture and set aside. Press to the bottom of the prepared pan. Bake for 20 minutes or until lightly browned.
In a medium sized sauce pot combine mixed berries, sugar, lemon juice, lemon zest, cornstarch and vanilla extract. Cook over low heat until thick. Set aside to cool slightly.
In another bowl, combine reserved crust mixture with chopped walnuts and cinnamon. Set aside.
Pour cooked berry filling into prebaked crust spread evenly. Dollop walnut crust on top of the berry filling.
Bake for 25-30 minutes or until top layer has set.
Cool slightly on a wire rack. Cut into squares and serve.