Preheat oven to 350 degrees F. Lightly grease, unless non-stick, a 9-inch square cake pan. Prepare shortbread base by mixing listed ingredients until mixture is crumbly. Transfer mixture to pan and press in to an even layer. Bake crust 12-15 minutes, until just golden at edges.
Prepare filling by whisking eggs with other listed ingredients until smooth. Pour filling over shortbread crust.
Reduce heat to 325 degrees F and bake until filling just sets, 25-30 minutes. Cool, and then slice into 16-20 squares. Dust with icing sugar before serving.