Preheat oven to 350°F. Butter the mini bundt pan and dust with cacao powder.
Whisk flour, salt, and baking soda together; set aside.
Cream butter and sugar. Add eggs one at a time, then sour cream and vanilla.
Mix in cacao powder and half the flour mixture. Add milk, then remaining flour — mix just until combined.
Divide batter among cavities and bake 15–18 minutes, until a toothpick comes out clean.
Cool in the pan 10–15 minutes, then turn out onto a rack to cool completely.
Microwave chopped chocolate and milk in 20-second bursts, stirring between each, until smooth. Stir in powdered sugar.
Glaze the cooled cakes and serve.