Preheat oven to 350 and line two 9" round cake pans with parchment paper and then grease sides with non-stick cooking spray, set aside.
In large bowl whisk together your four, sugar, baking powder, baking soda and salt until combined.
With hand or stand mixer mix in your eggs, oil and lemon juice until blended. Slowly add in your sour cream and continue beating until incorporated.
Fold in your poppy seeds.
Pour evenly into prepared pans and bake for about 25 minutes or until golden and cake springs back when touched, remove from oven and cool for 5 minutes in pans before inverting on wire rack to cool completely.
Meanwhile to make your buttercream in stand mixer or with hand mixer beat your butter until light and fluffy.
Add in you lemon juice and lemon zest and beat until blended.
Add in your powdered sugar one cup at a time until desired consistency is reached.
Add in milk if desired.
Taste flavor to adjust adding more lemon juice if needed,
To assemble, on a cake board on turntable place one layer of cake and spread with about 3/4 cup buttercream and spread out to sides.
Top with remaining layer of cake and then do a crumb coating by frosting top and sides of cake with a thin layer of buttercream.
Refrigerate for 10-15 minutes.
The frost remaining cake, reserving about 3/4 cup frosting for decorating if needed.
To make the swirl on top, using an offset spatula place tip in the center of cake and spin cake in a circle slowly moving your spatula out while spinning.