Fry bacon in a hot pan without any oil for 10 mins until crisp. Add chestnuts and fry until they are beginning to colour and coated in the fat. Pour away excess fat, then tip bacon and chestnuts into a bowl to cool.
Beat butter to soften with a spatula, then stir in the bacon and chestnuts. Season with black pepper, then store in fridge. This can be made up to 3 days ahead or frozen for 1 month. Defrost in fridge overnight.
To serve, place sprouts in a microwaveable bowl, pour in 100ml water, cover with cling film and pierce a few times. Microwave on High (850W) for 16-18 mins, stirring twice during cooking, or cook in a pan of boiling water for 5 mins. Drain and tip into a warm serving dish, then dot the butter over to melt. Toss through and serve.