For Chocolate Cake:Preheat oven to 325 degrees F. Lightly butter or grease the bottom and sides of an 8-inch round baking pan; line the bottom with a round of parchment paper.In a bowl, whisk together flour, sugar, cocoa powder, baking soda, baking powder and salt.In another bowl or 2-cup glass measuring cup, whisk together sour cream, oil, egg, and vanilla until evenly incorporated. Make a well in the center of the dry ingredients and pour in sour cream mixture. Stir until nearly combined and just a few streaks of dry ingredients remain.Pour in hot coffee or water and stir until smooth (it will start off very sloppy, but will incorporate eventually. Use a whisk if you’re having trouble.)Pour batter into prepared pan, spreading the batter into an even layer.Bake for 35 to 40 minutes or until center is springy and a toothpick inserted near the center comes out with just a few moist crumbs attached (no gooey batter). Remove from oven and let cool on a wire rack until pan is cool enough to touch, then invert on a wire rack to cool completely.For Frosting & Assembly:Combine cream and butter in a small saucepan. Stir gently over medium heat until butter is melted.Remove from heat and stir in chopped chocolate. Gently stir until completely melted (if the residual heat isn’t quite enough to melt the chocolate, return to a low burner for 30 seconds, then remove from heat and continue stirring).Add sugar, cocoa powder, and salt and whisk until incorporated; it will look like a slightly grainy brownie batter at this point.Add sour cream and vanilla and whisk until smooth and shiny. Set aside and let cool until slightly thickened and spreadable, about 15 to 30 minutes depending on the temperature of your kitchen, stirring occasionally to ensure even cooling.Generously spread frosting onto cooled cake, swirling with the back of a spoon or an offset spatula.Slice, serve, and enjoy! Cake will hold at cool room temperature for a few hours, otherwise refrigerate until ready to serve. Store leftovers in an airtight container in the refrigerator for up to 5 days.