Preheat the oven to 350 degrees F. Spray or grease two 9-inch round cake pans. Cut out 9-inch parchment paper circles, and press down in the bottom of each pan. (Trace the pan for perfect circles.)
For the Cake: In the bowl on an electric stand mixer, cream the butter and sugar together until light and fluffy. (Cream: Beat on high approximately 3-5 minutes to break down the sugar crystals.)
In a separate bowl, mix the cake flour, baking powder, baking soda, and salt.
Once the butter and sugar are well creamed. Scrape the mixing bowl with a rubber spatula. Then turn the mixer on low and beat in the eggs, oil, lemon zest, vanilla extract, lemon extract, and yellow food coloring.
Scrape the bowl again. Turn the mixer back on low and alternate adding the flour mixture and buttermilk until both are well combined. Turn off the mixer.
Pour the batter evenly into the two prepared pans. Bake on the middle rack, undisturbed for 30 minutes.
If the center of the cakes are puffed up, check the cake by inserting a toothpick into the center. If it comes out clean, take them out of the oven. If the toothpick has batter on it, bake another 5-10 minutes and recheck.
Allow the cakes to cool 10 minutes in the pans. Then carefully flip them out on a cooling rack. Remove the parchment paper and cool completely.
For the Frosting: Once the cakes are room temperature, place the butter in a clean stand mixer bowl. Beat until fluffy. Then scrape the bowl and turn on low. Mix in the buttermilk and lemon juice. Then add the powdered sugar one cup at a time, beating to incorporate.
After adding 5 cups of powdered sugar, turn the mixer up to beat out any clumps. Check the consistency of the frosting. If it is too loose, add another 1-2 cups of powdered sugar and beat until smooth.
To Assemble: Move one cake to a cake stand or platter. Scoop about 2 cups frosting onto the cake. Use a large flat spatula to spread the frosting out evenly.
Set the second cake on top of the frosting, being careful to center the cake. Gently press the cake a little to level the top. Then scoop all remaining frosting on top of the cake. Use the spatula to work the frosting over the top and down the sides.
Turn the cake stand to smooth out the sides with the flat edge of the spatula. Once you are satisfied with the sides, use the spatula to make swooping marks on the top of the cake. Use a wet paper towel to clean up the cake stand.
Cover and keep at room temperature for up to 4 days, or refrigerate for up to a week. When ready to serve, make sure the cake is at room temperature… All classic layer cakes taste best at room temperature!