In a large pot set over medium heat add water, 4 cups sugar, cardamom powder, and rosewater (if desired), and stir to combine. Cook until sugar has dissolved and syrup comes to a boil, then lower heat to keep at a simmer for 15 minutes, or until solution thickens. Remove from heat.
Remove prepared chhana from cheesecloth, and turn onto a clean surface or cutting board. Knead for about 15 minutes, to create a relatively smooth dough. When dough is smooth, divide into 8 equal portions, each about 1 inch in diameter. Form each into a ball.
Make a small indentation with your thumb in each ball, and place a pinch of sugar inside. Close up the indentation to conceal the sugar, and roll dough between your palms to reform each ball. (This step helps ensure the roshagullas stay moist inside.) Repeat with remaining balls. If wrinkles appear on the outside of balls, use a small amount of water to smooth them over.
Bring syrup back to a boil, then lower heat to keep at a simmer. Carefully drop in roshagullas, and cook for about 20 minutes. They will float and puff up slightly as they cook. Turn off heat, and allow to cool in the syrup. Roshagullas are delicious hot, but also may be served chilled or at room temperature.