In a medium saucepan, combine the water, butter, sugar, and salt. Bring to a gentle boil over low heat. Add the flour all at once, remove from the heat, and stir vigorously until a smooth dough forms and pulls away from the sides. Return to low heat and cook, stirring constantly, for 1 minute to lightly dry the dough. Transfer to a bowl and let cool completely.
Add the eggs one at a time, mixing thoroughly after each addition, until the dough is smooth, glossy, and elastic. Transfer to a piping bag fitted with a large star tip.
Lightly grease the outside of a muffin pan. Pipe the dough in tight spirals around each cavity to form cup shapes. Smooth any peaks with a damp fingertip. Freeze until firm, about 1 hour.
Heat the oil to 350°F/180°C. Release the frozen dough cups and fry until evenly golden and crisp, 3 to 4 minutes, turning as needed. Drain briefly, then toss immediately in the sugar mixed with cinnamon. Set aside.
To make the ganache, bring the cream just to a simmer. Pour over the chocolate, let stand for 1 minute, then whisk until smooth. Spoon the ganache into the churros cups and refrigerate until lightly set.
Whip the cream, mascarpone, vanilla, and powdered sugar to stiff peaks. Transfer to a piping bag fitted with a star tip. Pipe over the ganache. Garnish with fresh fruit, if desired, and refrigerate until serving.