Preheat oven to 170℃ (150℃ fan/340℉). Line a 23 x 23 cm (9 x 9 in) square tin or 29 x 19 cm (11.5 x 7.5 in) slice pan with parchment paper. See Note 1 for different tin sizes.
Combine the flour, oats, coconut, sugar, and salt in a large mixing bowl. Make sure to break up any clumps of brown sugar. Set aside.
Place the butter and golden syrup in small saucepan over low heat and stir until melted.
Dissolve the baking soda in the boiling water, then add it to the saucepan. The mixture will begin to foam, which is a good sign (it means your baking soda is active). Remove from the heat.
Make a well in the centre of the dry ingredients and pour in the warm butter mixture. Stir until just combined.
Pour the mixture into your prepared pan and press into an even layer. Bake for 18-22 minutes, or until golden brown around the edges and the middle is puffed and set. Cool the slice in the pan for 20 minutes. Then lift it out with the parchment paper and let it finish cooling on a wire rack.
Optional chocolate drizzle - Break the chocolate into pieces and place in a microwave-safe bowl. Microwave in 30-second bursts until melted, stirring in between. Or melt over a double boiler if you prefer. Drizzle over the cooled slice using a fork, a piping bag, or a small sandwich bag with the corner snipped. Allow the chocolate to set.
Cut into 12 rectangles or 16 squares, and finish with a sprinkle of sea salt flakes if desired.