Preheat oven to 350 degrees Fahrenheit. Prepare your Bundt pan by oiling and flouring the sides and bottom of the pan.
In the bowl of a stand mixer (or in a large bowl with a hand mixer), cream the butter and sugar until light and fluffy. Add sour cream and sourdough discard, mixing until well combined.
Add one egg at a time with the mixer on low, then add vanilla.
In a separate bowl, mix the flour, baking powder, baking soda, and salt.
With the mixer on low, alternate adding 1/3 of the dry ingredients and 1/3 of the milk until combined.
Pour into your prepared Bundt pan. Tap on the counter a few times to even it out.
Bake for 50 - 60 minutes or until a toothpick comes out clean. Let the cake cool in the pan for 15 - 20 minutes before inverting on a wire rack to cool completely.
Make the glaze by whisking the milk and powdered sugar until smooth, then drizzle over the top.
Notes
Don't pack the flour when measuring. A kitchen scale can be very helpful in reducing dirty dishes and having accurate measurements.
Be careful not to over-mix the batter, as this may affect the tenderness.
Most recipes will advise to oil and flour your Bundt pan to prevent sticking. This was unnecessary with my ceramic-coated Bundt pan. Oiling the pan worked better for me.
Let the cake cool for 15-20 minutes before attempting to remove it from the pan.
Glaze the cake on the cooling rack to keep your cake plate clean, or glaze on the cake plate so you don't waste any glaze.