Step 1

Adjust the oven rack to the center of the oven and preheat it to 350°F (180°C). Butter and flour a 10- or 12-cup Bundt pan.

Step 2

In a small saucepan over medium heat, combine the butter, cocoa powder, salt, and water. Cook, stirring, just until the butter is melted and everything is thoroughly combined. Remove from the heat and let cool slightly.

Step 3

Place the flour, sugar, and baking soda in a large bowl and whisk until blended. Add half the melted butter mixture and whisk until completely blended. The mixture will be quite thick. Add the remaining melted butter mixture and whisk until combined. Add the eggs, 1 at a time, whisking until completely blended before adding the next egg. Then whisk in the sour cream and vanilla until smooth.

Step 4

Scrape the batter into the prepared pan and bake until a toothpick inserted in the center of the cake comes out clean, 40 to 45 minutes. Let the cake cool in the pan for 15 minutes and then invert it onto a wire rack. Let cool completely before glazing.

Step 5

Just before glazing the cooled cake, place the chopped chocolate and agave nectar or corn syrup in a bowl. Combine the heavy cream and sugar in a small saucepan over medium heat. Stir constantly until the cream is hot and the sugar is dissolved. Pour the hot cream over the chocolate and whisk until melted and smooth. If the glaze is exceptionally runny, let it sit for a minute or so to thicken.

Step 6

Drizzle the glaze over the cooled cake, allowing it to drip and dribble down the sides. Wait a little, if you can, until the glaze is set prior to slicing and serving and helping yourself to seconds.