In a microwave-safe mug or ramekin, add all mug cake ingredients except lemon zest and mix until the batter is smooth.
Gently stir in 1/4 tsp lemon zest and add a little to garnish on top.
Cook the cake in the microwave for about 1 minute and 40 seconds or until the cake has set.
Let cake cool and optionally top with powdered sugar substitute, such as Swerve.