In a separate bowl, sift the egg and vigorously whisk or beat it until thoroughly combined (ensuring the yolk is fully incorporated with the white).
Next, add the sweetener and continue mixing until it's nearly completely dissolved.
Then, incorporate the milk and powdered milk, stirring until well blended.
Ensure that the mug is either cold or at room temperature before proceeding to the next step.
Now, carefully pour the prepared pudding batter (which should still be liquid) over the syrup in the mug, being cautious not to fill the mug beyond halfway.
Microwave on low power for 1 minute to 2 minutes and 30 seconds, pausing every 30 seconds to insert a fork. If the fork comes out clean, the flan is ready. Monitor the microwave closely; if the batter begins to rise and threatens to overflow, either reduce the power or the flan is likely cooked.
Allow the flan to cool within the mug to room temperature, then refrigerate until well chilled, ideally for at least 1 hour.
When ready to serve, carefully unmold your keto flan from the mug. If needed, gently run a small knife around the edges to loosen it from the mug before serving.