Preheat your oven to 350 degrees and line two baking sheets with parchment paper. In a medium bowl, whisk together the flour, baking soda, cream of tartar, and salt. Set aside.

In a large bowl, beat together the butter and sugar until light and fluffy, about 2-3 minutes with an electric hand mixer. Add in the egg yolks, vanilla, and milk and beat until combined. Add in the flour mixture and beat until fully combined, making sure you have no remaining flour clumps at the bottom of your bowl.

Dollop about half of the lemon curd and jam all over your dough in the bowl, and use a rubber spatula to gently fold and swirl it into the dough. Then use your spatula to flip the dough over, and repeat the dollop and fold process. You want to just barely fold in the curd and jam and leave big streaks visible throughout the dough.

Scoop the dough into 2-tablespoonful balls onto your prepared baking sheet and bake for 10-12 minutes, or until the cookies are set and just golden brown on the edges. Let the cookies cool for 5 minutes on the baking sheet, and then transfer to a baking rack to finish cooling. Enjoy!