Preheat oven to 350 degrees F. Spray the inside of a bundt cake pan or mini bundt cake pan with nonstick spray. Lightly flour the greased pan, gently shaking and tapping the pan around to distribute the flour as evenly as you can around the inside of the cake pan(s). Gently tap out excess flour over a sink.
Using a stand mixer or hand mixer, cream the butter on medium to high speed until smooth and creamy. Add sugar, then beat on high for 1-2 minutes, just until the sugar is nicely blended into the butter.
Add eggs and vanilla, then beat on medium-high until ingredients are well combined. Use a rubber spatula to scrap down the sides and bottom of the bowl as needed.
Beat in the orange zest, lemon zest, and orange juice until combined. Turn off the mixer.
In a large bowl, combine cake flour, salt, baking soda and baking powder.
With the mixer on low, slowly add alternating batches of the dry flour mixture and the whole milk (or non-dairy buttermilk) until the dough is formed and all ingredients are incorporated. Batter will be thick; be careful not to over-mix it!
Spoon batter into the bundt pan (or mini bundt pan). If using a mini bundt pan, leave about ¾ inch of space at the top for the dough to rise. You will also have enough batter for about 9-10 mini bundt cakes, so you'll be baking these in two batches.
Bake for 40-50 minutes, or when an inserted toothpick into the cake comes out clean. Remove from the oven and allow to cool slightly at room temperature, about 10 minutes, before gently tapping out of the pan and adding the glaze.
Make the Glaze
In a large bowl, whisk together the powdered sugar, orange juice, and lemon juice until you get your desired consistency.
Drizzle glaze immediately over slightly cooled cake(s). Slice, serve, enjoy!
Notes
To make a non-dairy buttermilk: add 1 tsp of lemon juice to ¾ cup almond milk. Allow to sit at room temperature for 5 minutes. Stir, and use as the buttermilk.Glaze tips: I like a slightly thicker glaze, so I'll sometimes add more powdered sugar. But if you like a thinner glaze, add 1 tsp of orange juice until you get your desired consistency. Storage: these mini bundt cakes will keep in an airtight container at room temperature for 4-5 days. You can freeze them in an airtight freezer-friendly storage container for up to one month.