Preheat the oven to 350℉. Spray a 9X9-inch square pan with baking spray. Line the bottom of the pan with parchment paper, spray again.
In a medium bowl, sift the flour, baking powder, baking soda, and salt. Set aside.
2 ¼ cups All-purpose flour, 2 ½ teaspoon Baking powder, ¼ teaspoon Baking soda, ½ teaspoon Salt
Using a mixer, beat the butter, oil, and sugar on high speed for 3 minutes until pale in color. Add in the lemon zest, lemon juice, and eggs. Mix on medium speed until combined. It will appear separated, but that is normal.
½ cup Unsalted butter, 3 tablespoon Oil, 1 cup White granulated sugar, Zest of 3 lemons, 2 tablespoon Lemon juice, 3 Large eggs
Add ⅓rd of the dry ingredients and half of the buttermilk and mix on low speed. Add another ⅓rd of the dry ingredients and the rest of the buttermilk. Then, finish with the rest of the dry ingredients.
1 cup Buttermilk
Spread the batter into the pan. Bake for 28-32 minutes until a toothpick inserted comes out clean.
Let the cake sit in the hot pan for 10 minutes before transferring a cooling rack. Let it cool completely before decorating.