>> time required: 4 hours
>> course: desert
>> serves: 18
>> source: ChefSteps - Nicholas Gavin

Heat oven to 149°C convection (or 163°C conventional), rack lower-middle. Spray a 20cm × 20cm #cake pan{} and line with a parchment sling.

In a #stand mixer{} with paddle, cream @butter{185%g}(softened to 18°C), @sugar{125%g}, @cornstarch{25%g}, and @kosher salt{4%g} on medium until pale and smooth, ~{4%minutes}.

Add @eggs{1}(yolk only, about 15g) and mix on medium until homogeneous, ~{2%minutes}.

On low, add @cake flour{225%g} and mix until no dry flour remains, ~{1%minutes}. Turn out onto a floured surface, press into a 12cm square, transfer to the pan, and press into the corners with an offset spatula until smooth and even.

Bake ~{1%hours}, rotating twice, until the top is light golden in the center and dark golden in the corners. Cover with overhanging parchment and press flat with a second pan or cardboard square. Cool ~{20%minutes} until firm.

In a #saucepan{}, combine @brown sugar{125%g}, @corn syrup{115%g}, @heavy cream{165%g}, @butter{10%g}, and @kosher salt{3%g}. Bring to a boil over medium and cook undisturbed ~{1%minutes}. Scrape bottom and sides, then continue cooking, stirring occasionally, until golden brown and 117°C, ~{5%minutes}.

Pour caramel over the shortbread, tilt to spread evenly, tap to remove air pockets. Cool uncovered ~{1%hours} until firm.

Temper @dark chocolate{200%g}(coarsely chopped) in a bain-marie: melt to 52°C, then cool at room temp stirring every 5 min until 28-29°C (about 30 min), then briefly rewarm in 5-second returns to the bain-marie until 31-32°C.

Pour tempered chocolate over the caramel, tilt to cover, tap to release bubbles. Cool uncovered at room temp at least ~{1%hours} (up to 24).

Lift out by the parchment to a cutting board. With a sharp knife dipped in warm water and wiped dry between cuts, slice into 6 bars, then crosswise into thirds for 18 portions.

Store in a parchment-lined airtight container at room temp up to 1 week; best within 3 days.