Prepare Caramel: Preheat the oven to 350°F (175°C) with a rack in the center position. Place the macadamia nuts and sesame seeds on a half sheet pan and toast for 8 to 10 minutes, stirring occasionally, or until golden brown. Cool on a wire rack.
Combine the sugar and water in a small saucepan. Stir over low heat until the sugar dissolves. Increase heat to medium and cook until the sugar reaches a medium to dark amber color (use a pastry brush to brush the sides of the pan with water to prevent crystallization). Remove from the heat. Carefully pour in the heavy cream and whisk vigorously to combine. Whisk in the butter and pinch of salt.
Add the toasted macadamia nuts and sesame seeds (reserving some for garnishing the sundaes) to the caramel and stir to coat evenly. Set aside to cool slightly while you assemble the sundaes.
Sundae Assembly: Once the caramel has cooled, drizzle on top of chocolate ice cream. Garnish with toasted coconut flakes as desired.
Inspired by the February 1994 issue of Bon Appetit magazine.