Lightly grease a 16-18 oz (approx 470-530 ml) microwave-safe mug or ramekin with butter or oil to prevent sticking.
Place the softened cream cheese into the mug. If it is not fully softened, microwave for 10-15 seconds.
Add the large egg and use a fork or small whisk to beat the egg into the cream cheese until the mixture is mostly smooth.
Add the sweetener, protein powder, ground flaxseed meal, almond extract, and cinnamon. Stir vigorously until all ingredients are fully combined and no dry lumps remain.
Microwave on high for 90-120 seconds (1.5 to 2 minutes). Cooking time will vary based on your microwave's wattage (e.g., 1000W microwaves may need closer to 90 seconds, while 800W may need the full 120 seconds).
The cheesecake is done when the edges are set and no liquid batter remains on the surface. For food safety, the internal temperature should reach 71°C (160°F). The center may still have a slight jiggle.
Let the mug cake rest for 1-2 minutes. For a firmer, more authentic "cheesecake" texture, allow it to cool for 5-10 minutes (in the mug or inverted onto a plate) before eating.