Preheat oven to 350 degrees. Butter an 8-inch round cake pan; line bottom with wax paper. Butter paper; dust with cocoa powder, tapping out excess; set aside.
In a medium bowl, sift together cocoa, flour, baking powder, and salt; set aside. In a mixing bowl, cream butter and sugar until light and fluffy. Beat in eggs, one at a time, then beat in vanilla. Reduce speed to low. Add flour mixture alternating with sour cream, starting and ending with the flour mixture.
Spread batter into prepared pan. Tap pan firmly on countertop several times to force out large air bubbles. Bake until a toothpick inserted in center comes out clean, 30 to 35 minutes. Remove from oven, Cool 10 minutes in pan, then invert onto a wire rack to cool completely (bottom side up). Set rack over a rimmed baking sheet. Pour glaze over; spread gently to coat entirely. Let stand until set. Garnish with chocolate shavings. Serve with whipped cream.