Combine flour, baking powder, baking soda and salt in a large bowl and whisk to blend evenly.

Combine soft butter, sugar, lemon zest and vanilla in a large bowl and beat with an electric mixer for 2 minutes until pale and fluffy.

Beat in egg until evenly incorporated. Mix in sour cream.

Add the flour mixture and fold it in by hand with a spatula until it is just combined.

Transfer the sticky dough to a piece of plastic wrap, flour your hands and carefully shape it into a 10-12 inch log. Wrap it well and refrigerate for 2 hours.

Preheat your oven to 375°F (NOTE: you will not bake at 375 for the whole time. You will need to lower the temperature to 350 after 3 minutes). Line two large cookie sheets with parchment paper.

Slice the log into 18 equal portions and roll each one into a smooth ball.

Roll each ball in granulated sugar and then roll in powdered sugar until evenly coated.

Place dough balls onto cookie sheets leaving at least 3 inches of space between them as they spread quite a bit.

Use a round ½-teaspoon measuring spoon to make an indent into the center of each dough ball.

Fill the indent with a heaped teaspoon of lemon curd.

Bake for 3 minutes, then lower the temperature to 350°F and bake for another 10 minutes until cracked and just lightly golden around the edges and the lemon curd looks matte.

Let cookies cool for 2 minutes on the baking sheets, then transfer them individually to a wire rack to cool completely.

Dust with icing sugar once they are cooled and serve!