Preheat oven to 325° F. Butter the bottom and sides of an 8x8 inch square cake pan.
In the bowl of a stand mixer, blend ¾ cup sugar and citrus zest until thoroughly blended, 1-2 minutes. Add remaining dry ingredients (1 ½ cups flour, ¾ tsp baking powder, ½ tsp baking soda, ¾ tsp salt); mix well.
Add 3 oz soft butter; mix until butter is broken down into fine crumbs. The mixture will look like soft sand.
Whisk together ¾ cup buttermilk, 2 eggs, and 2 tbsp citrus juice. With the mixer running on low, gradually add this mixture to the dry ingredients until blended.
Pour into prepared cake pan and bake until edges are golden and middle is baked through, about 35-40 minutes.
Allow to cool completely before frosting.