Combine 1 ½ cups sugar, ¾ cup butter/oil, 1 ½ tsp. vanilla, 4 egg yolks, and coconut milk in a pan over medium heat. Bring to a simmer; cook until thick, approx 12 minutes. Strain through a sieve into a bowl; stir in pecans and coconut; chill icing until firm.
Heat oven to 350° Grease three 20-22cm round cake pans with butter; line bottoms with baking paper circles. Grease paper; set aside.
Put chocolate into a small bowl; pour in ½ cup boiling water; let sit for 1 minute. Stir until smooth; set aside.
In a standing mixer, beat remaining butter/oil until fluffy (the oil won’t go as fluffy as butter); add remaining egg yolks one at a time. Add chocolate mixture and remaining vanilla; beat until smooth. On low speed, alternately add HGG baking mix and buttermilk until just combined; set batter aside.
Meanwhile, whip egg whites to soft peaks. Add remaining sugar; whip to stiff peaks. Fold egg whites into batter; divide between pans and smooth batter. Bake until cakes are set, 25–30 minutes. Let cakes cool; ice the top of each cake and assemble, leaving the sides bare.