For the Lemon Cake

• Preheat the oven to 350 degrees F and line a 9″ square baking pan with parchment paper.
• In the bowl of your stand mixer, massage the lemon zest into the sugar, then add the butter and oil and mix for 3 minutes on medium speed until well incorporated and fluffy in texture.
• Add the eggs one at a time until combined. 
• Scrape down the bowl to make sure no egg whites settled to the bottom.
• In a separate bowl whisk together the flour, cornstarch, baking powder, baking soda, and salt.
• In a large liquid measuring cup, whisk together the lemon juice, vanilla extract, lemon extract, water, and sour cream.
• Alternate adding the dry and wet ingredients, starting with the dry. Add half of the dry, all of the wet, then the remaining half of dry.
• Do an final mix by hand to ensure everything is mixed evenly, the pour the batter into the prepared pan.
• Bake at 350 for 28-30 minutes, or until a toothpick comes out mostly clean with a few moist crumbs attached. Let cake cool completely.

For the Lemon Cream Cheese Buttercream

• In the bowl of a stand mixer with the paddle attachment, cream the butter and cream cheese at medium high speed for 10 minutes. Make sure to stop the mixer every 2-3 minutes to scrape down the bowl.
• Add the powdered sugar 1/2 at a time, mixing on the lowest speed for 3 minutes after each addition.
• Add in the lemon zest and mix until combined. 
• Add the lemon juice 1 TBSP at a time until you reach your desired consistency.

For the Lemon Curd

• Whisk together all of the ingredients minus the butter and place on the stove over medium heat.
• Whisk constantly for 5 minutes, or until the mixture becomes bubbly and thick enough to coat the back of a wooden spoon. 
• Remove from heat and stir in the butter until melted and combined.
• Pour the curd through a fine mesh sieve to remove the lemon zest and any stringy bits of egg leaving the curd completely smooth.
• Press plastic wrap against the surface to prevent a skin from forming and chill until ready to serve.