Preheat your oven to 350°F (175°C) and line a 12-cup muffin tin with paper liners. In a medium bowl, whisk together the cake flour, baking powder, and salt. Set aside.

In a large bowl with a mixer, beat the room temperature butter until smooth. Add the granulated sugar and fresh lemon zest. Cream together on medium-high speed for 3-4 minutes until very light, pale, and fluffy.

With the mixer on low, add the eggs one at a time, mixing well after each addition. Scrape down the sides of the bowl as needed. Mix in the vanilla extract.

In a small bowl, whisk together the buttermilk and fresh lemon juice. Alternate adding the dry flour mixture and the wet buttermilk mixture to the batter, beginning and ending with the dry ingredients. Mix on low speed only until just combined after each addition. Do not overmix.

Remove the bowl from the mixer. Gently fold in the poppy seeds with a rubber spatula, scraping the bottom of the bowl to ensure everything is incorporated.

Fill each cupcake liner about two-thirds full. Bake for 18-22 minutes, or until a wooden skewer inserted into the center comes out clean. Let the cupcakes cool in the pan for a few minutes before transferring them to a wire rack to cool completely.

To make the frosting, beat the softened cream cheese and butter in a large bowl until completely smooth and free of lumps. Gradually add the sifted powdered sugar, mixing on low speed until combined, then increasing speed to beat until smooth.

Mix in the lemon juice, lemon zest, vanilla extract, and a pinch of salt until the frosting is light and creamy. If it's too thin, add more powdered sugar; if too thick, add a splash of milk.

Once the cupcakes are completely cool, frost them using an offset spatula or a piping bag. Garnish with extra lemon zest or poppy seeds if desired.