Preheat the oven to 350F and line baking sheets with parchment paper.
In a large bowl, using a hand mixer (or stand mixer), cream the butter and sugar together.
Add the egg yolks and lemon juice and beat until combined.
In a medium bowl, whisk together the flour and salt to remove any clumps. With the mixer running on low, gradually add the flour mixture until combined with the butter mixture.
Portion the dough into 38 small balls, making sure to leave 3” between the cookies on the sheet pan. I find it easiest to scoop the dough with a cookie scoop and then use my hands to roll the dough into balls before placing on the sheet pan. The dough is sticky so I recommend coating your hands in flour before handling the dough.
Coat your thumb in flour. Gently press your thumb into each cookie to make a "well." You'll most likely need to refresh your thumb with more flour to prevent it from sticking to the cookies. Don’t press all the way through or the cookies WILL break easily.
Fill each well (made with your thumbprint) with about ½ teaspoon of jam.
Bake the cookies for 12-15 minutes, until just beginning to brown on the edges. The cookies will have a lovely golden, yellow hue. That's the classic einkorn baked good color.
Let the cookies cool on the baking sheet for 5-10 minutes before carefully removing to a rack to cool completely. Do NOT move the cookies until they cool some or they will break. The jam and cookies need time to set.
Store the cookies in an air-tight container at room temperature for up to 3-4 days, or in the fridge for up to a week, or in the freezer for up to a couple of months.
Notes
Calorie Note: We don't believe in calorie counting for the average person eating nutrient-rich, real food (and these are a treat), but if you want to use the calorie information above, please know that the jam amount isn't calculated in the total calorie amount per cookie.