Preheat oven to 350 F and butter and line the bottom of a 8 x 8-inch baking pan with parchment paper.
Combine flour, salt, cornmeal and icing sugar. Cut in butter until the texture of coarse meal (dough will be crumbly).
Press dough into pan and bake for 15-18 minutes, until it just begins to colour around the edges. Allow to cool.
Whisk together all ingredients for filling until smooth and pour over crust.
Bake for 25-30 minutes, until set and a crust is visible on top of the squares.
Allow to cool at room temperature for 20 minutes, then chill for at least 2 hours.
Toss berries with limoncella and zest and set aside. Make no further than 2 hours in advance.
Prepare a pan with 2 inches of simmering water on the stove.
In a metal bowl, whisk together egg yolks, sugar, wine and lemon juice. Place the bowl over the pan of water, holding the bowl with a dry towel, and whisk vigorously until the mixture triples in volume and leaves a ribbon when the whisk is lifted. Remove from heat.
If preparing the sabayon in advance and you wish to chill it, whisk in the whipping cream and chill until ready to serve (up to a day).
Cut lemon squares and dust with icing sugar. Plate with berries and sabayon. Garnish with a sprig of fresh mint or lemon balm.