In a medium bowl, whisk together the self-rising flour, milk, sugar, egg, melted butter, vanilla extract, and ground cinnamon until the batter is smooth and well combined.
Gently fold the blueberries and white chocolate chips into the batter. If using, add the chopped macadamia nuts for a crunchy texture.
Divide the batter evenly between two microwave-safe mugs, ensuring each mug is no more than half full to allow room for rising. Microwave one mug at a time on high for about 1½ minutes, or until the top of the cake is firm but springy to the touch.
Let the cakes cool for a minute or two before serving. Enjoy as is or with a dollop of whipped cream or a drizzle of vanilla glaze.
Notes
Microwave wattages vary, so adjust the cooking time slightly if needed. Start with 1½ minutes and add 10-second increments if the cake isn’t fully set.