Preheat the oven to 325 degrees F. Line two 6-cup loaf pans, each 8 1/2 x 4 1/2 x 2 inches.
Following the melt-and-mix method, combine the butter, oil, sugar, chocolate, milk and 2/3 cup water in a large saucepan and stir over low heat until the sugar dissolves and the chocolate melts. Remove from the heat. Sift together the flour, cocoa, baking powder and baking soda into a bowl. Beat in the chocolate mixture (using a wire whisk means the mixture is less likely to form lumps).
Gradually beat the eggs, one at a time, into the chocolate mixture. Pour into the loaf pans and smooth the top with the back of a spoon. Place the pans on a baking sheet and bake for 50 minutes, or until a skewer comes out clean when inserted into the center of each cake. Cool in the pans for 5 minutes. Turn out onto a wire rack; peel off the paper and cool completely, crust side-up.
