For the chocolate pasta dough: Place the cake flour, all-purpose flour, cocoa powder, salt and egg yolks in a food processor. Pulse to combine. With the machine running, gradually add the oil, and then 1/3 to 1/2 cup water until the mixture forms a dough (the dough should stick together if pinched between your fingers). If necessary, add additional water, 1 teaspoon at a time, if the dough is too dry.
Place the dough on a lightly floured surface. Gather the dough into a ball and knead until smooth, about 5 minutes. Cover with plastic wrap and refrigerate for 30 minutes.
Cut the dough into quarters and press flat. Run each piece of pasta dough several times through a pasta-rolling machine, adjusting the setting each time until the pasta is about 1/8 to 1/16 inch thick. Cut the pasta into fettuccine noodles. Coat the cut pasta with cocoa powder to prevent sticking.
For the chocolate fettuccine with peas: In a small bowl, combine the mascarpone, Parmesan and vanilla and set aside.
Bring a large pot of salted water to a boil over high heat. Add the vegetable oil.
Place a large nonstick skillet over medium-high heat. Add the pancetta to the skillet and cook, stirring frequently, until golden and crisp, 6 to 8 minutes. Using a slotted spoon, remove the pancetta and drain on a paper-towel-lined plate. Add the butter and stir over medium heat until melted. Reduce the heat and simmer until frothy. Continue to cook until the butter has a nutty aroma and turns a caramel color, about 3 minutes. Add the sage leaves and cook until they are browned and toasted, about 2 minutes. Remove the fried sage to a paper-towel-lined plate. Keep the pan over low heat, stir in the peas and cook until they are warmed through, 2 to 3 minutes.
Meanwhile, add the pasta to the boiling water and cook until tender but firm to the bite, stirring occasionally, 2 to 3 minutes. Drain the pasta. Add the pasta, pancetta and most of the sage leaves to the skillet and toss gently to coat with the browned butter and peas.
To serve, add a dollop of the mascarpone mixture. Gently run a vegetable peeler over the chocolate to create chocolate curls. Sprinkle the curls over the pasta along with a few fried sage leaves.