In a small ceramic pot, bring the lemon juice to a boil, whisking while it heats. Let it boil for about 5 minutes, until it reduces to about a tablespoon.
Turn down the heat to medium low and add the butter, a few pieces at a time. Whisking until the butter has melted.
Remove from the heat and set aside.
Sift the flour, add the baking soda and salt, and set aside.
Place the egg and egg yolk in a large bowl, and using a hand mixer, beat until the egg has combined.
Add the sugar and beat until the mixture is creamy and pale.
While beating on low, drizzle in the melted butter mixture, until everything is blended.
Add half of the flour mixture until everything is blended, add the other half and mix only until everything is blended.
Cover the bowl with saran wrap and set in the fridge for an hour.
Cut the dough into 4 equal portions.
Place a portion on a piece of parchment paper, press flat with your hand. Place another piece of parchment paper on top and roll out to 1/8" thick. Slip the parchment onto a cookie sheet and repeat with each portion, stacking each piece on top of each other.
Place the cookie sheet into the fridge for 30 minutes.
Line an insulated baking sheet with a silpat pad and preheat the oven to 400°F.
Cut with a 2 1/2" cookie cutter, remove from the parchment paper, sprinkle with the sparkling sugar and place on the silpat pad about an inch apart.
Bake the cookies for 6 minutes to start, baking for an additional 30 seconds at a time, until the cookies are baked, but NOT brown.
Remove from the oven and let cool for about 5 minutes on the tray then move to a cooling rack.