Pulse all shortbread ingredients together in food processor until butter is the size of small peas. Press firmly into 9-inch square parchment-lined baking pan. Bake in preheated 350F oven for 20 minutes. Removed from oven and reduce heat to 325°F.
Beat all filling ingredients together. Pour over shortbread base and bake 30-35 minutes or until middle is almost set. Remove from oven and cool completely on wire rack. Chill if desired.
To serve, cut into squares and garnish with a light dusting of icing sugar, lemon zest, berries, and a sprig of mint. Can be served at room temerature.