Prepare pan: Grease a 9x9-inch cake pan; set aside.
In stand mixer bowl, add 1 1/4 cups flour, yeast, sugar and salt; mix well. In saucepan or microwave-safe dish, heat buttermilk, water and shortening to 120-130°F (shortening does not need to melt). Add to flour mixture. Using a paddle attachment, blend on low speed for 30 seconds. Add egg; beat on medium speed for 3 minutes.
Switch to dough hook attachment. Gradually stir in enough remaining flour to make a soft dough. Knead until smooth and elastic, about 3 minutes. Let rest 5 minutes.(NOTE: For hand mixing: following above steps, mix ingredients in large mixing bowl using a wooden spoon or dough whisk. Knead on floured surface until smooth and elastic.)
Prepare topping: Add all topping ingredients to a medium bowl; whisk together to combine. Set aside.
Press dough evenly in prepared pan. Sprinkle topping over dough. With back of spoon, make random indentations in dough. Cover; let rise in warm place until double, about 30 minutes.
While dough is rising, preheat oven to 375°F.
Bake for 25 to 30 minutes until golden brown. Serve warm or room temperature. Optional: garnish with additional lemon zest.