Heat oven to 350 degrees.
Grease a 9-by-13-by-2-inch baking pan well.
Whisk together 2 cups flour, 1 cup confectioners� sugar and salt in a medium bowl. Add the 1 cup of melted butter and stir until mixture has an even texture.
Transfer to the pan and pat to an even thickness.
Bake until golden brown, about 20 minutes.
Reduce heat to 300 degrees.
Whisk together eggs and granulated sugar until well combined. Add the remaining 1/3 cup flour and the verjus, and stir well.
Pour over shortbread and bake until just set, about 30 minutes.
Allow to cool completely in pan, then cut into 24 bars. Sieve the remaining 3 tablespoons of confectioners� sugar over the top before serving.
These bars will keep, covered and refrigerated, for about 3 days.