Preheat the oven to 325°F (163ºC). Prepare a 9x13 cake pan by spraying the sides with baking spray and lining the bottom with parchment paper.

In a jar or measuring cup, combine the lemon juice with the milk. Stir and set aside. It will curdle by the time you add it to the recipe at the end, and the curdling is the chemical reaction we want!

Add the sifted cake flour, baking powder, and salt into a bowl and whisk to combine. Set aside.

Using a stand mixer fitted with the paddle attachment, mix together the sugar and lemon zest until fragrant and well combined, about 1 minute. Add the butter and cream it together with the lemon-sugar mixture on medium speed until light and fluffy, 2-3 minutes.

With the mixer on low, add the egg whites one at a time. Add the vegetable oil and lemon extract (if using) and beat at high speed for one minute.

Turn the mixer off and add the cake flour mixture all at once. Mix on low speed until just combined, then slowly pour in the lemon-milk mixture. Mix until smooth and fully combined, about 30 seconds.

Pour the batter into the prepared cake pan and bake for 28-32 minutes, until a toothpick inserted into the center comes out clean or with a few moist crumbs. Cool completely to room temperature before frosting, either in the pan or carefully turn the cake out of the pan and onto a cooling rack.

In a stand mixer fitted with the paddle attachment, whip the butter on high speed until creamy and light in color, about 5 minutes.

Add the powdered sugar in two additions, mixing thoroughly for a minute and scraping down the bowl and paddle after each addition.

Add the lemon juice, whole milk and salt and mix on low until the buttercream is well incorporated and smooth, 1-2 minutes. Mix in the yellow food color gel, if using.

Once the lemon sheet cake has cooled, you can choose to decorate it in the pan (as pictured) or remove it from the pan for decorating. Use the lemon buttercream to frost the cake and garnish with fresh lemon slices.

To create the design pictured, use about ⅔ of the lemon buttercream to frost rustic swirls on the cake with an angled spatula. Add the rest of the lemon buttercream into a piping bag fitted with Wilton Tip 4B and pipe a shell border around the edges. Garnish with the lemon slices by fanning them out and placing them on opposite corners.