Assemble all of the ingredients so that they are within reach. In a small, heavy-bottom saucepan, place the granulated sugar and water and whisk to combine. Attach an instant-read candy thermometer to the side of the pan with the sensor in the mixture and not touching the side or bottom of the pan.
Bring the mixture to a boil over medium-high heat, and then turn the heat to medium-low. Cook, undisturbed, until the mixture reaches the soft ball stage (240°F). Remove the saucepan from the heat, and whisk the syrup until it becomes cloudy and almost creamy. Run the bottom of the saucepan under cold water to cool the syrup a bit.
Add the almond flour and the egg white to the syrup, and mix to combine. The mixture will be thick and clumpy. Return the saucepan to the stove and cook over low heat, stirring frequently, until the mixture melts and then thickens (if you run a spatula through the mixture, it will leave a trail).
Remove the saucepan from the heat and scrape the marzipan into a medium-sized bowl. Sprinkle the top with some confectioner’s sugar and allow it to cool briefly.
Once the marzipan is no longer too hot to handle, scrape it onto a flat surface sprinkled generously with confectioner’s sugar. Using a bench scraper, fold and knead the marzipan until it becomes smooth, dusting with more confectioner’s sugar as necessary to help it hold together.
Press the marzipan into a log or a ball, cover tightly with plastic wrap and place in the refrigerator until ready to use. It should keep well for about 2 weeks.