Preheat the oven to 200℃/400F. Roll the pastry out on a floured surface to make a 26 cm circle.
Spread 1/4 cup dulce de leche over the centre of the circle leaving a 3 cm (1 inch) border.
Melt the butter in a frying pan with the cardamom, and fry the bananas, cut-side down, for 1-2 minutes; you want them to look lightly cooked. Carefully turn them over and cook for another minute.
Lift the banana slices onto the centre of the tart and arrange them in a pattern. Scrape any residual butter and cardamom over and sprinkle with the sugar.
Brush the edge of the pastry with the egg, and bake the tart for 40 minutes, or until the pastry is puffed and golden and the bananas are caramelised.
If using thickened cream, beat the cream to soft peaks. If using double cream, it does not need beating. Fold in 1/4 cup dulce de leche to make a smooth toffee cream. Serve with the hot or room temperature tart.