Preheat oven to 350F/175C. Line an approximately 9 x 13 in (23 x 33cm) baking dish or roughly 1in/3cm deep baking sheet/tray lined with foil.
Melt the butter for the base then mix it with the graham cracker crumbs and sugar. Press the mixture in to the bottom of the lined dish/baking sheet in an even layer.
Peel, core and finely dice the apple and spread the pieces out on a baking sheet/tray lined with parchment.
Bake the crumb base and apples for approximately 8 minutes, until the base has firmed slightly and the apple pieces a softened and seem relatively dry. Leave the oven on.
Spread the apple pieces over the base then sprinkle the coconut evenly on top. Dot the dark and white chocolate chips evenly over the top then drizzle over the condensed milk.
Place in the oven and bake for approximately 20 minutes until the top is just starting to brown. Remove from the oven and let cool.
Gently warm the dulce de leche slightly, if it feels very thick, then put it in a piping bag and drizzle it in lines over the top of the bars.
Lift the block from the dish/baking sheet carefully by the foil then cut into pieces. (if it doesn't feel like it will come out of the dish as one without breaking, then cut in the dish/on baking sheet.)
Notes
These bars will keep for a few days in a sealed container, but they will soften slightly after the first day.