Line a 5-by-10-inch loaf pan with plastic wrap. Place chocolate in a heatproof bowl. Place yolks and a pinch of salt in another heatproof bowl.
Bring cream to a simmer in a medium saucepan. Slowly pour cream into bowl with yolks, whisking constantly; return mixture to saucepan set over low heat. Cook, stirring constantly, until mixture is thick enough to coat the back of a wooden spoon, about 8 minutes (mixture should not come to a boil). Immediately strain through a fine sieve set over chocolate; stir until chocolate melts and is smooth. Refrigerate, stirring occasionally, until thick, about 4 hours.
Spread 1 cup chocolate mixture evenly into bottom of loaf pan. Top with a layer of 4 graham crackers, trimmed to fit. Spread 1/2 cup chocolate over tops, and cover with some bananas. Spread 1/2 cup chocolate over bananas, and top with a layer of 4 trimmed graham crackers. Repeat with remaining chocolate, bananas, and graham crackers until you reach the top of the pan; finish with graham crackers.
Cover with plastic wrap, and refrigerate overnight. Uncover, and turn out onto a serving platter. Remove plastic wrap. Garnish with whipped cream, and cut into slices.
Unmolded icebox cake can be refrigerated for up to 2 days.