To make shortbread: in large bowl, cream together butter and both sugars until light and fluffy (5 min)
Beat in vanilla.
In medium bowl sift together both flours, baking powder and salt. With mixer on low speed, blend in dry ingredients.
Scrape dough unto plastic wrap and shape into 1/2" thick square. Refrigerate until firm (20 min). When chilled roll between two sheets parchment to an square slightly larger than 8"X8". Remove top parchment and press into 8"X8" baking pan with bottom parchment remaining under the shortbread.
Bake until golden brown at 350 º about 25-30 min. Remove from oven but keep heat set to 350º.
Make lemon curd: in medium sauce pan, heat lemon juice, butter and cream to just under a boil, butter should melt.
In medium bowl whisk eggs, sugar and yolks. Whisk in a bit of the hot liquid and gradually whisk in a bit more until all is added. Pour mixture back in the sauce pan and heat on medium, stirring constantly with wooden spoon, scraping sides and bottom of pan to keep eggs from scrambling. Cook until the curd coats the spoon thickly enough to leave a line when you draw a finger through it, 5-8 minutes. Remove from heat and strain through a fine sieve. Stir in salt and vanilla.
Pour curd over baked shortbread. Bake until curd has set and jiggles like firm jello, 15-20 min.
Let cool to room temperature.
Lift from pan using the bottom shortbread, refrigerated until completely set, 4 hours.
Cut into pieces and serve.