Crush cardamom seeds with the back of a heavy knife and combine, outer shells and all, with cream in a saucepan. Bring to a boil, cover and simmer for 10 minutes. In top of a double boiler whisk yolks with sugar until sugar dissolves. Cook eggs over simmering water, whisking constantly, until thickened. Remove from heat and gradually strain cream into yolks, whisking. Pour sauce back into top of double boiler and cook, stirring constantly, for 10 minutes until thickened. Temperature of sauce should not exceed 165 degrees F or it will curdle. Remove sauce from heat and continue to stir for 3 minutes to cool. Strain into a clean bowl and cool to room temperature, stirring frequently. Cover and chill.
In food processor combine banana and sugar and puree. Scrape puree into a bowl and gently fold in whipped cream. Cover and chill 1 to 2 hours. To serve, mound mousse in centers of 4 dessert plates. Pour some cardamom sauce around and serve immediately, garnished with mint.