Core the apple without cutting all the way through the bottom. Leave about ¼ inch intact to create a bowl effect for the filling. Remove the seeds and tough center, creating a 1-inch diameter hole.
Mix the filling in a small bowl by combining brown sugar, cinnamon, butter, and optional nuts and dried fruit until well incorporated.
Stuff the apple cavity with your filling mixture, packing it in well but not overflowing. The filling will melt and bubble during cooking.
Place the apple in a microwave-safe dish and add a splash of water to the bottom. This creates steam that helps cook the apple evenly.
Cover loosely with a microwave-safe lid or microwave-safe plastic wrap with one corner vented.
Microwave on high for 3-4 minutes until the apple is tender when pierced with a fork. Cooking time may vary depending on your microwave’s power and the apple’s size.
Let stand for 2 minutes before serving as the filling will be extremely hot and will continue cooking slightly.
Serve warm with a scoop of vanilla ice cream or a dollop of whipped cream if desired.