First, preheat your oven to 350°F and line a baking sheet with parchment paper
Whisk together the gluten-free all-purpose flour, almond flour, lemon zest, and a pinch of salt in a medium bowl.
In a large bowl, with a hand mixer or stand mixer, mix together room-temperature butter, sugar, maple syrup (optional), vanilla, and lemon juice. (do not be afraid if the maple syrup does not easily integrate into the butter. this is normal)
Then, slowly add in the flour mixture and mix with a rubber spatula until everything is combined. The dough should be nice and firm.
Scoop out tablespoons of dough, roll into balls, and place onto your prepared baking sheets. Gently press down to flatten them a bit, then use your thumb or the back of a teaspoon to make a little dent in the center of each cookie.
TO MAKE THE HEART SHAPE: I like to use the back of a 1/8 (one-eighth) teaspoon and make two small circles that join at the bottom. After making the initial two thumbprints, you can use a smaller finger to adjust the shape of the heart if needed.
Fill the cookies with jam and bake for 15 minutes, or until the bottoms turn golden brown. Let them cool on the baking sheet for about 5-10 minutes before transferring them to a wire rack to cool completely.