In a food processor, pulse together dried chamomile flowers, granulated sugar, and lemon zest until finely ground and fragrant, about 30 seconds.

In a large bowl, whisk together all-purpose flour, baking powder, baking soda, cornstarch, and salt until well combined.

Beat together unsalted butter and chamomile lemon sugar until light and fluffy, about 3–5 minutes. Add the egg and vanilla extract, mixing until fully incorporated.

Add sour cream (or Greek yogurt) to the butter-sugar mixture, blending thoroughly.

Gradually add dry ingredients to wet mixture, mixing just until combined.

Shape the dough into a ball, wrap in plastic wrap, and refrigerate for at least 1 hour.

Preheat oven to 350°F (180°C). Place oven rack in center position.

Roll out chilled dough to about ¼ inch thick and cut into desired shapes.

Bake cookies in preheated oven for about 10 minutes, or until edges turn golden.

Let cookies cool on baking sheet for a few minutes before transferring to a wire rack.

Whisk together powdered sugar, vanilla extract, milk, and lemon juice until smooth. Drizzle or spread icing over cooled cookies.