Preheat oven to 350˚F with a rack in the center of the oven. Butter the sides and base of two 9-inch cake pans and line the bottom of each pan with a ring of parchment paper.
In a large mixing bowl with an electric hand mixer, cream together 12 tablespoons of softened unsalted butter and 1 cup of granulated sugar for 2 minutes on high speed. Add eggs one at a time mixing just until incorporated.
In a small bowl, whisk together 2 cups of all-purpose flour, 2 1/2 teaspoons of baking powder, and 2 tablespoons of poppy seeds. Add half of the flour mixture, mixing just until incorporated.
Add 1 tablespoon of lemon juice, 1 tablespoon of lemon zest, and 1 teaspoon of vanilla extract, mixing just to combine.
With mixer on low speed, add 1/2 cup of buttermilk then mix in remaining half of the flour mixture, mixing until no lumps remain.
Divide between 2 prepared pans and spread evenly.
Bake at 350˚F for 20-23 minutes or until a toothpick inserted into the center comes out clean. Let cake cool in pans for 10 minutes then transfer to wire racks, remove backing, and cool to room temperature.
Once completely cooled, place the first cake layer on a cake platter and spread a generous 1/3 of the buttercream frosting over the top.
Place the second layer on top (flat-side-up) and spread 1/3 of the frosting over the top and sides.
Sprinkle the edges of the cake with poppy seeds if desired. Pipe remaining 1/3 of the frosting as desired over the top and place a slice of fresh lemon over each puff of frosting.
In a large mixing bowl with an electric hand mixer, beat 1 cup of unsalted butter on high speed until smooth and creamy (2 min). Add 4 oz of softened cream cheese and beat until completely smooth.
Add 4 cups of powdered sugar, 3 tablespoons of lemon juice, and 1 teaspoon of vanilla and beat on high speed until whipped and fluffy (5 min).
