Preheat oven and prep cake pan: Preheat oven to 350°F. Butter an 8-inch round cake pan; line bottom with a round of waxed or parchment paper. Butter paper; dust bottom and sides of pan with cocoa, tapping out excess.

Sift dry ingredients; cream butter and 1 cup sugar: Sift together cocoa, flour, baking powder, and salt. With an electric mixer on medium-high speed, cream butter and 1 cup sugar until light and fluffy.

Add eggs one at a time, then vanilla: Add eggs one at a time, beating well after each and scraping down sides of bowl as needed. Beat in vanilla.

Add flour mixture alternating with sour cream: Reduce speed to low. Add flour mixture in two batches, alternating with the sour cream and beginning and ending with the flour.

Spread batter in prepared pan and bake: Spread batter in prepared pan. Tap pan firmly on countertop once to eliminate large air bubbles. Bake until a cake tester inserted into the center comes out clean, 30 to 35 minutes.

Cool cake and glaze: Cool 10 minutes in pan, then invert onto a wire rack to cool completely (bottom-side up). Set rack with cake over a rimmed baking sheet. Pour glaze over cake, letting it run over sides; spread gently with an offset spatula or table knife to coat. Let set, about 30 minutes.

Whip cream and serve: If desired, whip cream and remaining 2 tablespoons sugar with an electric mixer until soft peaks form. Serve cake with whipped cream and chocolate shavings.