Step 1: Preheat And Prep
Preheat your oven to 350°F (175°C).
Grease a 10-12 inch cast-iron skillet with butter or cooking spray.
Step 2: Make the Cake Batter
In a medium saucepan, melt butter over low heat. Stir in cocoa powder and boiling water, then remove from heat.
In a large bowl, whisk together flour, sugar, salt, and baking soda.
Pour the chocolate mixture into the dry ingredients and mix until smooth.
Stir in sour cream, eggs, and vanilla extract until well combined.
Step 3: Bake the Cake
Pour the batter into the greased skillet and bake for 20-25 minutes, until the edges are set but the center is still slightly gooey.
Step 4: Make the Gooey Topping
While the cake is baking, melt chocolate chips with sweetened condensed milk in a saucepan over low heat, stirring until smooth.
Remove from heat and stir in vanilla extract.
Step 5: Finish And Serve
Remove the cake from the oven and immediately drizzle the gooey chocolate topping over the warm cake.
Let it sit for 5 minutes, then serve warm with vanilla ice cream or whipped cream.