In the bowl of a stand mixer with the paddle attachment, beat the butter and granulated sugar on medium speed until light and fluffy. Add the eggs one at a time, making sure they completely mix in before adding the next.
In a separate bowl, combine the flour, baking powder, lemon zest, and salt. In increments, add to the wet mixture with the mixer running until combined. Add the poppy seeds. Try not to overbeat.
Butter an 8" spring form pan and pour the batter in, smoothing the top. Place in an oven heated at 350 degrees Fahrenheit for about 25-30 minutes, or until the center is set and the edges are slightly golden.
For the glaze, whisk together the powdered sugar and lemon juice until smooth. If you want your glaze to be thicker, add more powdered sugar. To thin it out, add a bit of water or more lemon juice. Drizzle over the top of the slightly cooled cake.