Once the curd is cool and set, make the cookie dough. Preheat the oven to 350 degrees Fahrenheit (180 degrees Celsius) and line a baking sheet with parchment paper.
In a medium-sized bowl, whisk together the flour, cornstarch, baking powder and salt. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, or using a handheld mixer, beat the butter and cream cheese together on medium-high speed until smooth and no lumps remain. Add the granulated sugar and beat on medium-high speed until combined, about 2 minutes.
Add the egg, egg yolk and lemon extract and mix on low-medium speed until combined. Add the dry ingredients and mix on low speed until a dough forms. It will be thick and sticky.
Fill a small bowl with granulated sugar and another small bowl with powdered sugar.
Using a large cookie scoop, approximately 3 tablespoons, scoop the cookie dough and immediately place it in the granulated sugar. Toss to coat - immediately placing the scooped cookie dough into the granulated sugar will prevent the dough from sticking to your hands.
Roll the dough into a ball and toss in the powdered sugar - coat it generously. Place on the lined baking sheet, spacing the cookies about 3 inches apart. The cookies will spread in the oven.
Using a rounded tablespoon, press the bottom of the tablespoon into each cookie until it forms a round indent. Place a heaping teaspoon of curd into the indent. Bake the cookies for 11-12 minutes until the cookies are set and the curd is bubbling up. It will set when the cookies are cooling.
Let the cookies cool on the baking sheet for 5 minutes, then transfer to a cooling rack to cool completely. Dust with additional powdered sugar if desired. Enjoy!