Preheat oven to 350 °F (175 °C). Grease a mini muffin tin.

In a mixer bowl, cream butter, sugar, and lemon zest for 3–5 minutes until light and fluffy.

Add eggs one at a time, mixing on low and scraping between additions. Stir in vanilla and salt.

In a separate bowl, whisk together baking powder, baking soda, and flour. Add to wet ingredients and mix on low speed until just combined.

Scoop 1″ balls of dough into prepared mini muffin cups. Use the back of the scoop to press and create a small well in each.

Bake for 7–8 minutes or until edges are lightly golden. Cool in pan for 5 minutes, then transfer to a wire rack to cool completely.

Make the lemon curd: in a blender combine lemon juice, egg yolks, sugar, cornstarch, and lemon zest. Blend smooth. Pour into a saucepan and heat over medium, stirring constantly until it reaches a boil (~172 °F / 78 °C), then remove from heat and whisk in cold butter until smooth and incorporated.

When cookie cups are cooled, spoon about 1 tablespoon of warm lemon curd into each cup. Refrigerate until chilled.