Line an 8-inch square baking pan with parchment paper, leaving a few inches of excess paper hanging over the edges. Spray lightly with cooking spray and set pan aside.

Place the graham cracker crumbs, coconut, pecans, and salt in the bowl of a food processor and process until the mixture becomes fine crumbs.

Combine the butter and chocolate together in a heatproof bowl and microwave in 30-second intervals, stirring after each interval, until chocolate is melted and mixture is smooth.

Stir the graham cracker crumb mixture into the chocolate mixture and mix until well combined. Press mixture firmly into the bottom of the pan. Refrigerate until set, about 1 hour.

Place the butter, vanilla bean seeds, and vanilla extract in a small mixing bowl. Beat with an electric mixer set to medium until butter becomes very light in color. With mixer on low, gradually add the powdered sugar and custard powder and beat to combine—mixture will be very dry but this is fine; just keep going.

Beat in the milk and salt. Turn mixer to high and beat, stopping to scrape the bowl several times, until mixture is very thick and no lumps of powdered sugar remain. Spread the filling over the crust in an even layer and refrigerate to set, about 1 hour.

Melt the chocolate and butter in a microwave-safe bowl and heat in the microwave in 30-second intervals, stirring after each interval, until chocolate is melted and mixture is smooth. Stir in corn syrup until combined. Pour over the filling and spread with an offset spatula. Refrigerate bars for at least 3 hours, or overnight. Cut into bars and serve.