To make the cake:

Position a rack in the center of the oven and preheat to 350 degrees F. Brush two 8-inch round cake pans with oil, line the bottoms with circles of parchment paper, lightly brush the paper and pan with oil, and dust with flour.

Put the flour, baking powder, and salt into the bowl of a food processor. Add the sugar and orange zest and pulse a few times to combine evenly. Cut the butter into small pieces and add to dry ingredients. Pulse until the mixture looks like coarse sand, with some pea-size bits of butter, about 5 times.

Whisk the milk, eggs, and vanilla together in a liquid measuring cup. With the processor running, pour in the wet ingredients and process to make a smooth batter (this takes less than a minute). Divide the batter between the prepared pans and bake until a toothpick inserted in the center comes out clean, about 25 minutes.

Cool the cakes on a rack for 15 minutes. Then run a knife around the edges of the pans, turn the cakes out onto a plate, and peel off the paper. Flip the cakes upright and cool completely on the rack, about 45 minutes. (This is important-icing a warm cake is not a happy thing).

When ready to ice the cake, make the frosting:

Put the chocolates in a medium microwave-safe bowl. Microwave on medium for a minute, stir, and repeat until chocolate melts, about 3 minutes in all. (Or put chocolates in a heatproof bowl set over an inch or so of water at a very slow simmer; set the bowl on the pan; don't let the bowl touch the water. Stir occasionally until smooth.)

Let chocolate cool slightly. Stir the sour cream, coffee, and vanilla into the chocolates. Beat with a handheld electric mixer until frosting is silky and fluffy. (Spread frosting soon after making it; it firms up at room temperature.)

Set a large flat plate on a large inverted bowl or bottom of a salad spinner (of course, if you have a cake stand, use that). Dab a little frosting in the center of the plate and place a cake layer, top side up, on top. With an offset spatula, scoop about one-third of the frosting onto the cake and spread it evenly to the edge of the cake. Place the other layer on top and press down lightly. Spread another third of the frosting around the sides. Spread the remaining frosting on the top of the cake. Lightly touch the frosting with the back of a spoon to make swirly peaks. Serve immediately or set aside at room temperature for up to 2 hours before serving. If refrigerating the cake, bring to room temperature 30 minutes before serving.