Combine 1 cup water, the jelly and 2 tablespoons elderflower liqueur in a saucepan over medium heat; bring to a boil, then reduce the heat to low and simmer until syrupy, 20 to 25 minutes.
Whisk the flour, sugar, baking powder, salt and nutmeg in a large bowl. Stir in the cream, eggs, vanilla and the remaining 3 tablespoons elderflower liqueur until smooth.
Heat about 1/3 inches vegetable oil in a large pot over medium heat until a deep=fry thermometer registers 350 degrees F. Working in batches, drop heaping tablespoonfuls of the batter into the oil and fry, turning, until golden, 4 minutes. (Reduce the heat if the doughnuts are browning too quickly.)
Remove with a slotted spoon and drain on paper towels, then roll in cinnamon sugar.
