Ingredients
1/2 cup (1 stick / 113 g) unsalted butter, cut into pieces
1 (18–20 oz) box brownie mix (family-size, for a 9×13-inch pan)
1 1/2 cups whole milk
1/2 cup granulated sugar
1/4 cup unsweetened cocoa powder
Directions
Prepare the slow cooker: Lightly grease the inside of a 4- to 6-quart slow cooker with butter or nonstick spray. Scatter the pieces of unsalted butter evenly over the bottom of the crock.
Make the brownie base: In a large bowl, whisk the boxed brownie mix with 3/4 cup of the milk (reserve the remaining 3/4 cup for the sauce) until just combined. The batter will be fairly thick. Pour or spoon this batter directly over the butter in the slow cooker, spreading it gently into an even layer without stirring the butter in too much.
Mix the fudge topping: In a separate bowl, whisk together the granulated sugar and cocoa powder until no cocoa lumps remain. Stir in the remaining 3/4 cup milk to form a smooth, pourable chocolate mixture.
Layer the fudge mixture: Slowly pour the cocoa-sugar mixture evenly over the brownie batter in the slow cooker. Do not stir. The layering is intentional—during cooking, the liquid mixture will sink and form a hot fudge sauce while the brownie batter rises and sets on top.
Cook the pudding: Cover the slow cooker with the lid. Cook on HIGH for 2 to 2 1/2 hours, or on LOW for 3 1/2 to 4 hours. The top should look set and cakey around the edges but still slightly soft in the center. When you insert a spoon and lift gently, you should see a thick, glossy fudge sauce underneath.
Rest and serve: Turn off the slow cooker and let the pudding rest, covered, for about 10–15 minutes to allow the sauce to thicken slightly. To serve, spoon down through the cakey top layer to scoop up both brownie and hot fudge from the bottom. Serve warm, with ice cream or whipped cream if you like
Variations & Tips
For a deeper chocolate flavor, use dark chocolate or “fudge” style brownie mix and replace 1/4 cup of the milk with strong brewed coffee or espresso; the coffee won’t make the dish taste like mocha, but it will intensify the cocoa. If you prefer a slightly less sweet dessert, reduce the granulated sugar in the sauce layer to 1/3 cup and use a natural cocoa powder instead of a Dutch-processed one. Add texture by sprinkling 1/2 to 3/4 cup chopped toasted walnuts, pecans, or hazelnuts over the brownie batter before you pour on the fudge mixture. For a peanut butter twist, dollop 1/3 cup warmed peanut butter over the brownie batter and lightly swirl it in with a knife. You can also introduce a flavor theme with extracts: a teaspoon of vanilla is classic, but almond extract pairs nicely with cherries, and peppermint extract works well for winter holidays (add crushed peppermint candies on top after cooking). For serving a crowd, keep the slow cooker on the WARM setting and place a clean tea towel under the lid to catch condensation, so the top stays pleasantly cakey instead of soggy. Leftovers reheat well in short bursts in the microwave; if the sauce seems too thick the next day, loosen it with a spoonful of milk or cream before warming.