Melt butter gently without boiling; chill in fridge for about 20 minutes until cooled to room temperature.
In a bowl, rub granulated sugar and lemon zest together until textured like wet sand to release flavor.
Whisk cooled butter with lemon sugar mixture using a spatula or mixer for 1 minute.
Add egg and vanilla extract, mix until fully combined.
In a separate bowl, combine flour, baking powder, baking soda, poppy seeds, and salt.
Add dry ingredients to wet ingredients and gently fold with a spatula until just combined.
Using a 2 tbsp cookie scoop, portion dough into 11 balls; roll and chill on a baking tray for 30 minutes.
Preheat oven to 355°F (180°C). Place 6 cookies on a lined baking sheet, keep the rest chilled.
Bake the first batch for 10-11 minutes, then let cool on tray for 3 minutes before transferring to a wire rack to cool completely.
Repeat baking with remaining cookies.
For frosting, whip cream cheese, vanilla, powdered sugar, agave or honey, and heavy cream using an electric or stand mixer until stiff peaks form.
Fill a piping bag fitted with a star tip and pipe frosting in swirls on cooled cookies.
Garnish with poppy seeds and lemon slices if desired.