In a separate small bowl, mix the stout, 50ml brandy, and beaten egg.
Pour the wet mixture into the dry ingredients. Stir until you have a stiff, sticky batter.
Spoon the mixture into the prepared pudding basin. Cover the top with a pleated piece of parchment paper, then cover that with a layer of aluminum foil, crimping the edges securely around the rim of the basin. Tie string around the rim to secure the covering.
Steam the pudding in a large saucepan or slow cooker with boiling water covering at least two-thirds of the basin. Steam for 5 to 6 hours, checking the water level regularly and topping up with boiling water as needed.
Once cooked, carefully remove the basin from the water. Let it cool slightly, then remove the foil and parchment. Place a serving plate over the basin and invert to release the pudding.
To make the Brandy Butter Sauce, beat the softened butter and powdered sugar together until light and creamy. Beat in the 2 tablespoons of brandy until smooth.
Serve the warm plum pudding immediately, topping each slice with a spoonful of the brandy butter sauce. If desired, warm a little extra brandy and carefully pour over the pudding before lighting it for a festive presentation.
Notes
You can prepare this traditional plum pudding up to a month ahead and store it wrapped tightly in the refrigerator or freezer. Reheat by steaming for 2 hours before serving.
For a less traditional but easier method, cook this recipe in a slow cooker on low for 8 hours.
A silver sixpence, if you have one, can be wrapped in parchment paper and placed in the center of the pudding before steaming for good luck.