Place the butter, water, cocoa powder, and salt in a small saucepan. Cook over medium heat, stirring frequently, until the butter has melted and the mixture is combined. Set aside.

Whisk together the flour, sugar, and baking soda in a large mixing bowl. Add half of the butter mixture and whisk to combine. Add the remaining butter mixture, and whisk until combined. Add the eggs, one at a time, whisking well after each addition. Whisk in the sour cream and vanilla.

Pour the batter into the prepared pan and spread evenly. Bake 40 to 45 minutes, or until a pick instead into the center comes out clean.

Cool the cake in the pan for 15 minutes. Then invert the cake onto a wire rack to cool completely.

Place the chocolate and corn syrup in a medium bowl.

Combine the cream and sugar in a small saucepan. Cook over medium heat, stirring occasionally, just until the sugar has dissolved and the cream begins to bubble around the edges.

Pour the cream mixture over the chocolate. Let stand for 5 minutes. Then whisk until smooth. Drizzle over the cooled cake.