In a bowl, whisk together flour, baking powder, salt and chopped thyme. Set aside.
In a separate bowl, beat butter and sugar until light and fluffy.
Mix in egg, vanilla extract, and lemon zest until fully incorporated.
Gradually add dry ingredients to the butter mixture, mixing just until a soft dough forms.
Wrap the dough and chill for atleast 1 hour.
Preheat oven to 350 F. Scoop or roll dough into 1 inch balls. Roll lightly in sugar and place on parchment lined baking sheet. Gently flatten.
Bake 10-12 minutes, until edges are set and bottoms are lightly golden. Centers should still be soft. Let cool in pan for a few minutes and then transfer to a rack.