Spray and line three 8 or 9-inch round cake pans with parchment paper. Be sure to spray the sides of the pans. Preheat the oven to 350°F conventional.

Zest just the bright yellow parts of the lemon and mix with the granulated sugar. Rub the zest into the sugar with your fingers to help the sugar begin to pull out the oils.

In a large bowl, whisk together flour, baking powder, baking soda and salt; set aside. 

In the bowl of a stand mixer fitted with the paddle attachment or in a separate large bowl with an electric mixer, cream butter, vegetable oil, sugar, and vanilla until light and fluffy.

Add the eggs one at a time, beating on medium speed to incorporate. Scrape down the sides of the bowl after each addition.

In a measuring cup, combine milk with the lemon juice. This will ease incorporating the wet and dry ingredients. Alternate adding flour mixture with the milk mixture. I like to begin and end with dry ingredients. Mix just until the flour is incorporated. Stir in poppy seeds.

Divide batter evenly among 3 prepared pans. Bake on the center rack until a cake tester toothpick or bamboo comes out with only a few clinging crumbs but no wet batter streaks, approximately 25-30 minutes.

Cool 10 minutes in the pans. Then, turn out onto a cooling rack to cool completely upside down.

In the bowl of a stand mixer fitted with the paddle attachment or in a large bowl with a hand mixer, cream ¾ cup softened butter, cream cheese, and powdered sugar until smooth.

Add the lemon juice and lemon zest. Beat well.

Stir in poppy seeds until just combined.

On your favorite cake stand, cake plate or platter, place the first layer of cake. Top with half a container of lemon curd (about 5 ounces); spread in an even layer to within a 1/2-inch of edges. Add about 1 cup of the frosting on top, and spread out in an even layer with an offset spatula.

Place the second layer on top and add the remaining curd along with another cup of frosting. 

Top with the third cake layer. Frost the top of the cake and spread it out with a large offset spatula, allowing it to fold over the edges.

Smooth a thin layer of buttercream over the sides of the cake. It will be wiggly-jiggly, so this will be a little challenging. You could end here with a naked cake or you can pop the whole cake in the refrigerator for an hour or two until the frosting has set.

Spread the remaining buttercream over the sides and top. I swirl the top using my cake stand like a turn-table and the tip of my large offset spatula. Then I smoothed the edges using a bench scraper. Refrigerate covered, until ready to serve.