Preheat oven to 350°F. Butter or spray 8 miniature cake pans. Place on top of rimmed baking sheet. Bring 1 cup stout and 1 cup butter to simmer in heavy large saucepan over medium heat. Add cocoa powder and whisk until mixture is smooth. (The mixture seemed separated, but it worked just fine.) Cool slightly.
Whisk flour, sugar, baking soda, and 3/4 teaspoon salt in large bowl to blend. Using electric mixer, beat eggs and sour cream in another large bowl to blend. Add stout-chocolate mixture to egg mixture and beat just to combine. Add flour mixture and beat briefly on slow speed. Using rubber spatula, fold batter until completely combined. Pour batter evenly into prepared pans (about 3/4 full). Bake cakes until tester inserted into center comes out clean, about 30 minutes. Transfer cakes to rack; cool completely in the pans, then turn cake out onto rack for drizzling ganache.
Pour chocolate chips into a heat-proof bowl. Heat the cream and butter in a sauce pan over medium heat until bubbly around the edges. Pour directly into the bowl with the chocolate. Let set for a minute, then whisk until smooth and creamy. Pour over individual cakes and decorate as desired.
Cake recipe adapted from Smitten Kitchen. Ganache recipe from Pennies on a Platter.