Add the pumpkin puree, mascarpone, ½ cup of sweetener, and vanilla extract to a bowl. Using a hand mixer with whisk attachments, beat on medium speed until fully incorporated. Set it aside.
Separate egg whites from yolks. Add the eggs and egg yolks to a bowl with the prepared mixture. Beat with a hand mixer on medium speed until smooth.
Add the heavy cream, ½ cup at a time, beating constantly on medium-low speed. Make sure not to overmix the flan mixture. Set it aside.
Place one cup of sweetener and water in a saucepan and bring to a boil. When the syrup starts to color, reduce the heat. Cook until the caramel turns golden brown.
Meanwhile, grease the ramekins with some butter. Place them in a deep roasting tin. Add enough hot water to the roasting tin to come halfway up the sides of the ramekins. Remove the caramel from the heat and divide it between the ramekins, swirling it around the sides. Use about one tablespoon of the caramel per ramekin. Carefully pour the flan mixture into the caramel-lined ramekins. Depending on the size of your roasting tin, you will need to cook the flan in batches.
Preheat the oven to 175°C (350°F). Cook the flan for 35-40 minutes. The exact cooking time will depend on the size of your ramekins, so keep an eye on it after about 30 minutes. The flan is ready when the edges are set and firm, and the center of the flan is slightly jiggly. Don’t overcook the flan to avoid the bubbles from forming.
When done, remove from the oven and cool completely. Use a knife to gently go around the edges of the flan to help it release more easily. Refrigerate for a few hours before serving.