Preheat your oven to 350F/175C. Grease a large (12 cup capacity) bundt pan well with butter and set it aside (see notes).
Combine the buttermilk and limoncello in a measuring jug and set aside.
Peel and core the apples and chop then into ½" pieces. Sprinkle the apples with a few tablespoons of the measure out all-purpose flour and toss them to coat.
Mix together the remaining flour along with the baking powder and baking soda and salt and set aside.
In a large bowl, cream together the butter and sugar with either a spatula or wooden spoon until very light and fluffy. When the mixture is light and fluffy, add the in lemon zest, eggs and vanilla extract and mix well to combine.
Into this, add in a third of the flour mixture and mix lightly before adding in half of the buttermilk + limoncello mixture. Mix just until combined. Repeat this process again and finally add in the last ⅓ of the flour mixture and mix just until combined. Don't over mix this batter.
Finally, add in the flour coated apples to the batter and fold them in.
Pour the batter in the prepared pan and bake it for about 60-75 minutes or until fully cooked through. Insert a skewer into the center of the cake to check if it is fully done. When there is no wet batter on the skewer, it is fully cooked.
Allow the cake to cool completely before inverting it onto a platter.