Preheat oven to 350 and prepare a 12½ x 17½ baking pan (standard cookie sheet) by greasing it.

Combine butter, water, and cocoa in a medium-sized sauce pan. Stir and heat until the butter is completely melted and the mixture comes together.

Remove pan from the heat and stir in sugar, vanilla, salt and sour cream; mix until well combined. Stir in eggs and continue to mix until combined.

In a small bowl, combine flour and baking soda. Gently fold the flour mixture into the batter and mix just until no dry spots remain. Try not to over-mix as this will cause the cake to be rubbery.

Pour the batter into the prepared baking sheet, spread evenly and bake for about 17-20 minutes or until a toothpick inserted into the center comes out clean or with just a few moist crumbs.

While the cake is baking, combine butter and brown sugar in a sauce pan and bring to a simmer. Let cook for about 1½-2 minutes, stirring constantly and remove from heat.

Using a whisk, stir in cream or milk, vanilla, salt, and powdered sugar, mixing until smooth.

Spread over the WARM cake and top with crushed pretzels. The pretzels are best when put on the same day you eat it or they lose their crunch. I usually just put pretzels on the pieces we're immediately eating.