Preheat oven to 350˚F (180˚C). Grease and line bottom and sides of 9-in. (23-cm) springform pan. In bowl, stir together crushed candy bars, coconut, graham cracker crumbs, cocoa powder, sugar and salt. Drizzle with butter and egg, stirring until combined. Press into prepared pan. Bake 10 to 12 min. until firm. Cool in pan on wire rack.
Meanwhile, in bowl, using electric mixer on medium speed, beat together butter, custard powder and vanilla until smooth and creamy, about 1 min. With mixer on low, beat in icing sugar in thirds, alternating with 2 additions of milk. Scrape bowl as needed between additions. Add up to 2 tsp (10 mL) additional milk if filling is too thick to spread. Spread over cooled crust, smoothing top. Sprinkle candy bar chunks on top and press into filling. Refrigerate 1 hr. until firm.
Place chocolate chips and butter in heat-proof bowl set over saucepan of barely simmering water 5 min. until melted. (Note: Chocolate chips can retain shapes even when melted; stir to avoid scorching.) Spread melted chocolate over filling, smoothing top. Sprinkle with salt. Refrigerate 1 hr. until chocolate is set. Remove cake from pan. Cut into slices and serve. Refrigerate in airtight container up to 4 days or freeze up to 2 weeks.
Substitute broken chocolate wafer cookies for the Coffee Crisp® candy bar chunks.