Preheat oven to 350F. Spray the bottom and sides of a 9-inch cake pan with pan spray and line bottom with parchment paper. Set aside.
Place eggs in a large mixing bowl (or in the bowl of a stand mixer fitted with whisk attachment) and beat on medium-high speed about 3 minutes until eggs are thick and yellow.
Add sugar, lemon zest, lemon juice, citrus liqueur, and milk to egg mixture and beat until smooth. With mixer running on low, slowly drizzle in olive oil until all the oil has been added and mixture is smooth.
In a large separate bowl, combine flour, cornstarch, baking powder, baking soda, salt, and poppyseeds. Gently fold flour mixture into liquid until batter is just combined.
Scrape batter into prepared pan and smooth top. Bake cake at 350F 35-40 minutes until a toothpick inserted in center comes out clean. Allow cake to cool completely in pan on a wire cooling rack.
Once cake is cooled, run a knife around pan edges and invert cake onto a serving plate. Just before serving, dust cake with powdered sugar. Slice into wedges and enjoy!