To make the sugar cookie crust, in a standing mixer (or by hand) beat the butter and granulated sugar until light and fluffy, about 3 minutes oh high. Add the egg and mix on medium until fully incorporated. Add the flour, baking powder, and salt all at once and mix on medium just until no spots of flour remain, about 20 seconds (do not overmix). Wrap the dough in parchment paper and refrigerate for 30 minutes.
Preheat the oven to 350 degrees F. Lightly grease a mini muffin tin with butter or cooking spray.
While the oven is preheating, make the lemon filling by whisking together the eggs, egg yolks, whipping cream, lemon juice, and granulated sugar until smooth.
Remove the cookie dough from the refrigerator. Divide the dough into quarters and divide each quarter into six even balls of dough. Press each ball of dough it into a space in the muffin tin. Using your thumbs, press the dough into the tin so that it is evenly distributed along the sides and bottom. (Don't worry if it is a bit thinner in some places than others.) Pour the lemon filling into the center of each sugar cookie cup, stopping just below the top of the crust.
Bake until the filling is set and the crust begins to turn golden, 10-12 minutes. (Do not over bake - pull the tarts out just when you see any golden color on the crusts.) Allow these to cool completely at room temperature. Refrigerate until ready to serve.
Before serving, dust the tops of the tarts with 2 Tablespoons of powdered sugar. Beat the whipping cream with the remaining 2 Tablespoons of sugar until stiff peaks form. Pipe or spoon the whipping cream on top of each tart. Serve chilled.