In a saucepan, melt the 8 oz chocolate squares, 4 tbsp butter, oil and water. Do not let it come to a boil. Stir until smooth.
In a separate bowl whisk together the eggs, 2 tsp vanilla and sour cream until smooth and creamy.
In another bowl, sift together the flour, sugar, salt and baking soda. With an electric mixer, combine the chocolate mixture with the dry ingredients. After that beat the egg mixture in to create a smooth batter.
Pour the batter into a greased 18x13 inch rimmed baking sheet.
Bake in a 350F oven for 18-20 minutes. Test to make sure a toothpick inserted in the centre comes out clean.
Prepare the icing halfway through the baking of the cake. In a saucepan, heat the remaining butter, heavy cream, cocoa powder and corn syrup. Stir often and once smooth, remove from the heat. Stir in the icing sugar and vanilla to create a smooth, creamy icing.
Once the cake is baked, remove it from the oven and immediately pour and spread the icing all over the cake. Sprinkle the chopped pecans on top.
Let the cake cool completely, so the icing can set, before serving (1-2 hours).