Preheat the oven to 350°F. Grease and line three 6-inch round cake pans with parchment paper and set aside.

In a medium bowl, combine the granulated sugar and lemon zest. Rub them together between your fingers until the sugar is fragrant and slightly damp, which helps release the oils from the zest.

In the bowl of a stand mixer fitted with the whisk attachment, add the softened butter and vegetable oil. Beat on medium speed for about 2 minutes, until smooth and creamy.

Add the lemon sugar to the butter mixture and beat on medium-high speed for 2-3 minutes, until light, fluffy, and pale with a pearlescent sheen.

Reduce the mixer speed to medium-low and add the eggs and egg white one at a time, mixing well after each addition. Stop to scrape down the sides and bottom of the bowl as needed to ensure everything is evenly incorporated.

In a separate bowl, whisk together the flour, baking powder, salt, and poppy seeds. Add the dry ingredients to the batter and mix on low speed just until no dry streaks remain. Be careful not to overmix, as this can lead to a dense cake.

Divide the batter evenly between the prepared pans. Bake for 20-24 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs.

Let the cake layers cool in their pans for about 10 minutes, then turn them out onto a wire rack to cool completely before assembling.

Place the egg whites and powdered sugar in the bowl of a the mixer. Beat the ingredients on low for 30 seconds.

Increase the speed to high and beat for 8-10 until a meringue forms that is thick and opaque with tips that stand up but bend at the ends. If the meringue doesn't fully set, that's ok. As long as it's thick and pure white, the frosting will come together.

Turn the speed down to low and add the softened butter 1 tablespoon at a time. Mix on low for about 10 minutes.

Stop the mixer and switch the wire whip for the paddle attachment. Beat in the vanilla extract and lemon curd on medium-low to remove air bubbles.

Place one cake layer on a cake stand. Pipe a tall ring of buttercream around the edge to create a barrier, then fill the center with an even layer of lemon curd.

Place the second cake layer on top and repeat the buttercream border and lemon curd filling. Top with the final cake layer upside down for a flat surface. Spread a thin crumb coat of buttercream over the entire cake to lock in any loose crumbs.

Chill the cake in the freezer for about 20 minutes, or until the crumb coat is firm. Finish by spreading a thick, smooth layer of buttercream over the cake. Transfer any remaining frosting to a piping bag fitted with a decorative tip and pipe swirls or designs on top as desired.