1. Preheat the oven to 180°C (350°F). Grease and flour a 23cm (9-inch) round baking pan.

2. In a medium bowl, whisk together the flour, baking powder, glucomannan powder, and saffron.

3. In a large bowl, beat the eggs and sugar with an electric mixer until light and fluffy. Mix in the vanilla sugar.

4. In a separate bowl, whip the cream until stiff peaks form. Gently fold the whipped cream into the egg mixture.

5. Gradually add the flour mixture to the egg mixture, stirring until just combined.

6. Add the softened butter to the batter and mix until smooth.

7. Pour the batter into the prepared baking pan and smooth the top with a spatula.

8. Bake for 35-40 minutes, or until a toothpick inserted into the center comes out clean.

9. Remove the cake from the oven and let it cool on a wire rack for at least 30 minutes before removing it from the pan.

10. Once the cake is completely cool, frost and decorate as desired. Serve with your favorite tea or enjoy it on its own.