Preheat the oven to 375° F. Coat a 9-inch loaf pan that is not non-stick and is, preferably, light colored, with cooking spray. Set the pan aside.
Sift the cocoa onto a square of wax paper. Set the cocoa aside.
In a medium mixing bowl, use an electric mixer on medium-high speed butter, sugar, and vanilla for 3 minutes. Beat in 1 egg until well combined. Add the remaining egg and beat for 4 minutes. Blend in the sour cream. Mix in the baking powder, baking soda, and salt. Add the sifted cocoa, reduce the speed to low and mix to combine. Add in the flour and mix until the flour is just blended in. Do not over-mix; leaving some white streaks produces a more moist and tender cake. The batter will be dense, resembling chocolate mousse. Spread the batter evenly in the prepared pan and smooth the top with a flexible spatula.
Bake for 50-60 minutes, or until a straw inserted into the center of the cake comes out clean. Cool the cake in the pan on a wire rack for 30 minutes. Take the cake out of the pan and cool it completely on the rack. Its flavor will be even better the second day. Sour Cream Chocolate Loaf keeps for 4 days if wrapped in foil and kept on the counter.
To serve, cut the cake into 12 slices with a serrated knife. Serve with tea, or for dessert, topped with sliced fresh strawberries and a dollop of whipped cream, or a scoop of ice cream.