In a large mixing bowl, blend the softened butter and sugar until the mixture turns pale and fluffy. Beat in the eggs, followed by the sour cream, mixing until smooth.

In a separate bowl, whisk together the flour, cocoa powder, baking soda, and salt. Gradually incorporate the dry mixture into the wet ingredients, stirring just until the dough is evenly combined.

Preheat your oven to 350°F (175°C) and prepare a baking sheet by lining it with parchment paper. Drop generous spoonfuls of the dough onto the sheet, spacing them slightly apart. Bake for 8 to 10 minutes, or until the edges are firm and the tops develop light cracks. Keep the cookies on the baking tray once removed from the oven.

While the cookies are still warm, begin making the glaze. In a small saucepan over medium heat, combine the butter, cocoa powder, and milk. Stir continuously until the mixture begins to simmer. Remove from heat and whisk in the powdered sugar and vanilla extract until a smooth, pourable icing forms.

Drizzle or spoon the warm glaze over the cookies while they are still slightly hot. Allow them to rest at room temperature until the icing sets into a delicate, crackled finish.