MAKE THE CAKE Preheat the oven to 350 degrees and grease a Bundt pan with butter and flour. (Non-stick spray will not cut it!)
In a medium sized bowl, whisk the flour, baking powder, baking soda and salt until well combined.
In a stand mixer fitted with the paddle attachment, cream the butter, olive oil, sugar and lemon zest until light and fluffy, about 3 minutes.
Add the eggs, one at a time, allowing each to incorporate before adding the next.
Add the vanilla extract and mix to incorporate.
In a 1-cup liquid measuring cup, combine the buttermilk and lemon juice.
With the mixer on low, add half the dry ingredients to the butter mixture and mix until streaky.
Drizzle in the buttermilk mixture and mix on low until it is almost completely combined.
Add the remaining dry ingredients and mix on low until well combined; do not overmix.
Spoon the batter into the prepared Bundt pan and smooth the top with a small offset spatula.
Bake until golden brown, and a cake tester comes out clean, for 45 to 50 minutes.
Cool in the pan for 20 minutes before flipping onto a cooling rack; cool completely.
MAKE THE GLAZE In a medium-sized bowl, stir the sugar, butter, limoncello and milk to create a smooth glaze.
Spoon the glaze over the top of the cake, immediately, allowing it to drip down the sides. Allow the glaze to set before serving.