A moist, tender Bundt cake with rich, buttery vanilla notes and a subtle tang from sourdough discard, finished with a simple, silky glaze.
Preheat oven to 350°F (175°C). Grease and flour the sides and bottom of a Bundt pan.
In a stand mixer (or large bowl with a hand mixer), cream butter and sugar until light and fluffy. Mix in sour cream and sourdough discard until well combined.
With mixer on low, add eggs one at a time, then mix in vanilla.
In a separate bowl, whisk together flour, baking powder, baking soda, and salt.
With mixer on low, alternate adding one-third of the dry ingredients with one-third of the milk until just combined. Do not overmix.
Pour batter into the prepared Bundt pan and tap the pan on the counter a few times to level.
Bake for 50–60 minutes, or until a toothpick inserted into the center comes out clean.
Cool in the pan for 15–20 minutes, then invert onto a wire rack to cool completely.
For the glaze, whisk powdered sugar with enough milk to reach a smooth, pourable consistency, then drizzle over the cooled cake.