For the cake: Preheat an oven to 350 degrees F. Grease a half sheet pan and dust with cocoa powder to prevent sticking or line with parchment paper and lightly grease the paper and sides.

In a medium saucepan, combine the guinness, butter and chocolate and warm over medium heat, whisking until the chocolate and butter are melted. Allow to cool slightly.

In a mixing bowl, combine the flour, sugar, cocoa powder, baking soda, and salt and whisk to combine everything. In a small bowl, whisk together the eggs and sour cream. Add the slightly cooled chocolate mixture to the flour mixture and whisk to combine. Whisk in the egg and sour cream mixture until smooth. Pour the cake batter into the prepared pan and bake for about 22-25 minutes, until the cake is set and a tester comes out clean.

For the sauce: While the cake is baking, combine the milk, guinness, sugar, and cocoa powder and whisk until smooth. Bring to a gentle simmer and add the butter, chocolate, vanilla and salt and whisk until smooth. Set aside. Once the cake comes out of the oven, use a bamboo skewer to poke holes all over the warm cake. Pour the chocolate glaze over the warm cake, working slowly to prevent overflow and using an offset spatula or the back of a spoon to smooth it to an even layer. The cake will absorb the chocolate sauce slowly.

For the pecans: Preheat an oven to 350 degrees F. In a medium bowl, whisk together the egg white and sugar until frothy. Add the ground cinnamon and whisk until smooth. Toss the pecans in the egg whites until evenly coated and spread on a baking sheet lined with a silicon mat. Bake for 15 minutes, stirring the pecans every 5 minutes until they look dry. Cool and then top the cake with the pecans.