Best Anna Olson's Nanaimo Bars

Ingredients

• 1 cup graham cracker crumbs
• 3 tbsp cocoa powder
• ½ tsp salt
• 1 cup sweetened flaked coconut
• ½ cup walnut pieces
• 6 tbsp unsalted butter, melted
• 1 large egg, lightly beaten
• ½ cup unsalted butter, at room temperature
• 2 cups icing sugar, sifted
• 2 tbsp vanilla custard powder
• Pinch of salt
• 3 tbsp milk
• 1 tsp vanilla extract
• 4 ounce semisweet chocolate, chopped
• 2 tbsp unsalted butter
• Sea salt, for sprinkling (optional)