Preheat the oven to 180°C/350°F (160°C fan). Line a 27.5cm × 17.5 cm slice tin with baking paper, leaving a little overhang.
Add the butter and golden syrup to the Thermomix bowl. Melt for 2 min/70°C/speed 2. Scrape down the sides of the bowl, then melt for a further 1 min/70°C/speed 2 until smooth.
In a small bowl, mix the bicarbonate of soda with the boiling water. Add to the Thermomix bowl and mix for 5 sec/speed 3.
Add the flour, oats, coconut, caster sugar and brown sugar. Mix for 20 sec/reverse/speed 3, until combined.
Transfer the mixture to the prepared tin, spreading it evenly over the base before pressing down firmly. The back of a spoon or the flat base of a measuring cup works well for this.
Bake for 20-25 minutes, or until lightly golden and just set in the centre. Mine took 25 minutes, but baking times can vary between ovens. The slice may still feel slightly soft in the centre when removed from the oven, but it will firm up as it cools.
Leave to cool in the tin for 20 minutes, then lift out and transfer to a wire rack to cool completely. Once cool, slice into bars or squares.
Notes
Recipe tips: For detailed tips, troubleshooting advice and helpful baking notes, refer to the full post above.Storage: Keep the slice in an airtight container at room temperature for up to 1 week. Freeze for up to 3 months. Allow to thaw at room temperature before serving.