Preheat the oven to 350°F (175°C). Grease the mini bundt pan with baking spray or melted butter.
Mix dry ingredients: In a medium bowl, whisk together flour, baking powder, baking soda, and salt. Set aside.
Cream butter and sugar: In a large bowl, beat the butter and sugar until light and fluffy (about 2–3 minutes).
Add eggs and flavorings: Beat in eggs one at a time, then mix in vanilla extract and lemon zest.
Combine wet and dry: Alternately add dry ingredients and sour cream to the batter, starting and ending with dry ingredients. Stir in the lemon juice just until combined. Do not overmix.
Fill and bake: Spoon batter into the greased mini bundt pan cavities, filling each about ¾ full. Bake for 15–18 minutes, or until a toothpick inserted comes out clean.
Cool: Let cakes cool in the pan for 5 minutes, then invert onto a wire rack to cool completely.
Make glaze: Whisk powdered sugar, lemon juice, and milk until smooth. Adjust consistency if needed.