Put the rhubarb and butter in a microwaveable bowl and cook for 30-40 seconds (800 watt) until softened. Set aside.
Mix together the flour, bicarbonate of soda, sugar and rhubarb in a bowl.
Add the egg and milk and mix well.
Divide between 2 large microwaveable mugs. Sprinkle with pistachios.
Microwave separately for 1 1/2 to 3 minutes each (800 watt) until risen and firm.
Microwave cooking times vary widely so the cake needs to be watched all the time as it cooks.
Depending on your microwave it could be ready in 1 minute or it could take up to 3 minutes.