Preheat oven to 350˚F. Spray one 8-inch round cake pan (with 2-inch sides) with baking spray and line it with parchment paper. (If your cake pan is lower than 2 inches, use a 9-inch pan.)
Stir the soft butter in a medium-large bowl until creamy. Add the oil, sugar and vanilla . Stir until well combined and creamy, about 1 minute.
Sprinkle the baking powder, the salt and approximately one-third of the flour over the butter mixture. Stir just until the flour has disappeared.
Add half of the milk and stir to combine. Add another third of the flour and stir just until incorporated. Repeat, adding remaining milk and then the remaining flour, stirring after each addition until almost completely combined before adding the next. Stir in the poppy seeds.
Whisk egg whites in a separate clean bowl for 1 ½-2 minutes by hand or with a hand mixer until foamy and a peak forms when you lift the whisk or beater out of the egg whites.
Using a spatula, gently fold the egg whites into the cake batter. Scrape the sides and bottom of the bowl to make sure everything is mixed in, but don’t over-mix. Transfer the batter to the prepared pan and smooth the top.
Bake for 35-40 minutes, or until a toothpick inserted in the center of each cake layer comes out clean or until the center reaches 210˚F with an instant thermometer.
Cool the cake in the pan for 15 minutes. If the cake looks like it’s stuck on the sides of the pan, run a thin knife around the edge, then invert it onto a cooling rack to cool completely before filling and icing.
With a long, serrated knife, slice the cake in half horizontally and place bottom half on a cake stand or serving platter. Scoop the lemon curd (¾-1 cup, depending on how thick you like the fillling) right onto the center of the bottom cake layer. Spread the lemon curd to within a ½-inch of the sides of the cake.
Set the other layer gently on top of the lemon curd, cut side down.
Frost the cake as desired with the lemon curd icing. Serve and enjoy!