Mississippi Mud Cake

Preparation

1. For the Cake

   Step 1

   Preheat the oven to 350°F/180°C/Gas Mark 4). Grease a 9" × 13" (23 × 33 cm) baking pan with softened butter, then dust with flour.

   Step 2

   In a small saucepan, combine 16 Tbsp. (8 oz./225 g) unsalted butter, cut into chunks, and ½ cup (40 g) unsweetened cocoa powder and cook over medium heat, stirring often, until smooth, 2–3 minutes.

   Step 3

   In a large bowl, whisk together 2 cups (400 g) granulated (caster) sugar, 1½ cups (195 g) all-purpose (plain) flour, ⅛ tsp. fine salt, 4 eggs, beaten, and 1 tsp. pure vanilla extract. Add the chocolate/butter mixture and whisk to incorporate everything into a thick, smooth batter. Stir in 1 cup (125 g) chopped pecans, then quickly pour the batter into the pan.

   Step 4

   Bake for 25–30 minutes, until the cake springs back when touched gently in the center and is beginning to pull away from the sides of the pan.

2. For the Frosting and Topping

   Step 5

   In a large bowl, with an electric mixer, beat together 4 cups (480 g) powdered (icing) sugar, ½ cup (40 g) unsweetened cocoa powder, 8 Tbsp. (4 oz./115 g) unsalted butter, melted, ½ cup (4 fl oz./120 ml) whole milk, and 1 tsp. pure vanilla extract on medium speed until incorporated, creamy, and smooth, 2–3 minutes.

   Step 6

   When the cake is done, remove it from the oven, scatter 4 cups (10 oz./285 g) mini marshmallows over the top, and return the cake to the hot oven to soften the marshmallows, about 3 minutes.

   Step 7

   Place the hot cake on a wire rack. Pour the frosting over the marshmallow-dotted cake, and let it cool to room temperature.

   Do Ahead: Store airtight at room temperature for up to 5 days or in the refrigerator for up to 1 week.