How to Make It
In a medium bowl, whisk the flour with the coconut, baking soda and salt. In a standing electric mixer fitted with a paddle, or using a hand mixer in a large bowl, beat the butter with the sugar at medium speed until light and fluffy, about 3 minutes. Beat in the egg yolk and vanilla, scraping the side and bottom of the bowl. Beat in the dry ingredients at medium-low speed. Wrap the dough in plastic and refrigerate for 15 minutes.
Preheat the oven to 350°. Position the racks in the upper and lower thirds of the oven. Roll the dough into 1-inch balls and bake for 14 minutes. Make a dent in the cookies; fill with jam and bake for about 10 minutes. Let the cookies cool on the sheets for 5 minutes, then transfer to racks to cool completely.