Cast Iron Hot Fudge Pudding Cake

6 tablespoons unsalted butter, cut into 6 pieces
2 ounces bittersweet chocolate, choppoed coarse
2/3 cup (2 ounces) unsweetened cocoa powder
34/ cup (3 3/4 ounces) flour
2 teaspoons baking powder
1/4 teaspoon salt
1/3 cup packed (2 1/3 ounces) light brown sugar
1 cup (7 ounces) sugar
1 cup brewed coffee
1/2 cu water
1/3 cup whole milk
1 tablespoon vanilla estract
1 large egg yolk

Instructions
Adjust oven rack to iddle position and heat oven to 325 degrees. Melt buter, chocolate, and 1/3 cup cocoa together in 10-inch skillet over low heat, stirring oten until smooth, 2 to 4 minutes. Set aside to cool slightly.

Whisk flour, baking powder, and salt together in bowl.  In separate bowl, whisk brown sugar, 1/3 cup sugar, and remaining 1/3 cup cocoao together , breaking up any large clumps of brown sugar with fingers. In thir bowl, combine coffee and water.

Whisk milk, vanilla, egg yolk, and remaining 2/3 cup sugar into cooled chocolate mixture.  Whisk in flour misture until just combine.  Sprinkle rown sugar mixture evenly over top, covering entire surface of batter. Pour coffe mixture gently over brown sugar mixture.

Transfer skillet to oven and bake until cake is puffed and bubbling and just beginning to pull away from sides of skillet, about 35 minutes, rotating skillet halfway through baking. Using potholders, transfer skillet to wire rack and let cake cool for 15 minutes. serve