This luscious cake is the official state dessert of Massachusetts.
FOR THE CAKE: Preheat oven to 350 degrees. Oil and flour two 8-inch round cake pans. In a medium bowl, whisk together flour, baking powder and soda, and salt. In a large bowl, whisk together the oil and sugar. Whisk in eggs and vanilla. Alternately add in flour mixture and sour cream, beginning and ending with flour. Divide batter between pans. Bake for 30-35 minutes, until cakes pull away from the side of the pan. Cool in pans for 10 minutes. Transfer to a wire rack to cool completely. Set aside.
FOR THE PASTRY CREAM: In a large bowl, lightly beat the egg and yolks together. Add the sugar, a little at a time. Continue beating until mixture falls in ribbons when the beater is lifted, about 5 minutes. Mix in the flour and milk in a steady stream. Transfer the mixture to a saucepan. Boil gently over medium heat, cooking and whisking until the mixture thickens, 10 minutes. Strain the mixture though a fine sieve into a large bowl. Stir in the vanilla. Press plastic wrap over the surface. Cool for at least 1 hour. (To chill cream quickly, place bowl over bowl of ice and stir until cooled.)
FOR THE CHOCOLATE TOPPING: Place chocolate in a medium bowl. Bring cream to a simmer in a small saucepan. Pour cream over chocolate, and leave undisturbed 1 minute. Stir completely and allow to cool for about 6-8 minutes.
ASSEMBLY: Slice off the domed top of one cake to level. Hollow out 1/2-inch depth with fingers, leaving a 1/2-inch border from the edge. Place cake on a plate or cake stand. Fill with the pastry cream. It's okay for cream to spill a bit over edges. Top with the second cake, dome side up. Pour the chocolate topping over the cake and let it drip over the sides. Chill until ready to serve, at least 5 hours or up to overnight.