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Cream the butter and sugar well.
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Beat in the egg yolks and lemon extract.
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Stir in the lemon zest.
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Fold in the flour and salt until fully incorporated and a soft dough forms.
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Wrap the dough in plastic wrap and chill for about an hour.
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Preheat the oven to 350 degrees F.
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Roll the dough into 1 inch balls and roll the balls in coarse sugar. I like turbinado. (this is an optional step)
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Place the dough balls on a parchment paper lined baking sheets, about 2 inches apart.
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Using your finger or the handle of a wooden spoon make a deep indentation in the center of each dough ball.
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Bake for 16-18 minutes until they start to appear golden at the bottom edges.
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Remove from the oven and spoon blueberry jam into the centers to fill the little wells.
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Return the cookies to the oven for 3-4 minutes.
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Transfer the cookies to a wire rack to cool completely. Store in airtight containers.
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These will freeze well in a single layer in a plastic container.