Preheat the oven to 350 degrees Fahrenheit.
In a medium bowl, add the softened butter and sugar. If the butter was previously in the fridge, add to the microwave for about 20 seconds to soften before adding the sugar to the bowl. You want the butter to be soft to the touch but not melted.
Cream together the butter and sugar with an electric mixer until smooth. Add in the egg, vanilla, lemon juice and lemon zest. I suggest zesting two lemons first and then using one for the juice.
Beat together with the electric mixer until well combined. Add in the flour, salt, baking powder and baking soda. Be sure to measure the flour using the spoon and level method, do not measure straight from the bag.
Beat just until a dough forms.
Use a wooden spoon to fold in the poppy seeds until evenly incorporated.
Use a medium cookie scoop to drop the dough in even circles onto a baking sheet lined with parchment paper. Leave as is and don't flatten to ensure the cookies are fluffy. Bake for 10-12 minutes. 11 minutes is perfect in my oven, but all ovens are different. You want the bottoms/edges to be very slightly golden brown, but the center should be very soft and almost appear a bit raw in the cracks when you remove from the oven. Careful not to over bake as they will dry out easily!
Allow to cool for 10 minutes on the baking sheet and then transfer to a wire rack to finish cooling.
Once completely cool, whisk together the powdered sugar and lemon juice until creamy. If it's too thin, add a bit more sugar, too thick and add more lemon juice. Drizzle over the cookies and sprinkle with extra poppy seeds if desired.