Melt the butter in a microwave-safe bowl or stovetop until fully liquid. Let it cool to room temperature.

In a mixing bowl, combine granulated sugar with freshly grated lemon zest. Rub the zest into the sugar for about a minute.

Add the cooled melted butter to the lemon-sugar mixture, then incorporate one large egg and vanilla extract; mix until smooth.

In a separate bowl, whisk together flour, baking powder, baking soda, poppy seeds, and salt. Gradually add this dry mixture to wet ingredients.

Scoop dough into balls, using about a tablespoon of dough each. Place them on a parchment-lined baking sheet and refrigerate for 30 minutes.

Preheat your oven to 180ºC (355ºF).

Remove chilled dough from the refrigerator. Bake cookie balls for 10-11 minutes until edges are lightly golden and centers appear soft.

While cookies cool, prepare the frosting by beating cream cheese, powdered sugar, vanilla, agave or honey, and heavy cream until stiff peaks form.

Once cookies are cooled, frost each cookie and sprinkle with additional poppy seeds and fresh lemon slices if desired.