Lightly grease the insert of a 4- to 6-quart~ slow cooker with 4 tbsp unsalted butter melted and slightly cooled or Cooking Spray. Line the bottom with a piece of Parchment Paper if desired for easier cleanup.

In a Mixing Bowl, whisk together the 1 cup all-purpose flour, ¾ cup granulated sugar, ¼ cup of the ½ cup unsweetened cocoa powder divided, 1 ½ tsp baking powder, and ¼ tsp fine salt. Add the ½ cup whole milk room temperature, melted 4 tbsp unsalted butter melted and slightly cooled, and 1 tsp pure vanilla extract, and stir with a rubber spatula until a thick, smooth batter forms. Spread the batter evenly into the bottom of the prepared slow cooker insert.

In a small Mixing Bowl, stir together the ¾ cup granulated sugar and remaining ¼ cup ½ cup unsweetened cocoa powder divided. Scatter the chopped 4 oz bittersweet chocolate 60–70% cacao, roughly chopped evenly over the batter, then sprinkle the ¾ cup granulated sugar–0.5 cup unsweetened cocoa powder divided mixture on top. Carefully pour the boiling~ 1 ½ cups boiling water over everything — do not stir. The layers will look messy; that's exactly right.

Place a double layer of Paper Towel under the lid to absorb condensation and prevent dripping onto the cake. Cook on HIGH^ for 2 to 2½ hours~, until the top of the cake looks set and matte but the center still jiggles slightly when you gently shake the insert. Do not lift the lid during the first 2 hours~.

Turn off the Slow Cooker and let the cake rest uncovered for 15 minutes~ — the fudge sauce will thicken slightly as it cools. Scoop into bowls, making sure to get some of the molten hot fudge from the bottom. Dust with ¾ cup granulated sugar and serve immediately with a scoop of vanilla ice cream.