How to Make It
Preheat the oven to 350°. Line a large rimmed baking sheet with parchment paper. In a medium bowl, mix the coconut with the turbinado sugar, egg whites, vanilla and salt. Transfer the macaroon mixture to the prepared baking sheet and pat it into a 12-by-4-inch rectangle 1/2 inch thick. Bake for 25 minutes, or until golden around the edge. Let cool.
Meanwhile, in a medium saucepan, combine the granulated sugar and water and cook over moderately high heat until reduced by a quarter. Add the chestnuts and rum and cook over moderate heat, stirring occasionally, until the chestnuts are coated in a thick syrup, about 15 minutes. Pour into a bowl and let cool.
In a medium glass bowl, combine the chocolate and cream and melt in a microwave oven on high for 1 minute, stirring halfway through.
Transfer the macaroon rectangle to a platter and arrange the chestnuts on top in 3 rows of 12. Pour the chocolate over the entire top of the macaroon rectangle. Let stand at room temperature for 30 minutes, or until set. Slice into bars and serve.