Whisk together flour, cocoa powder, sugar and a pinch of salt in a bowl.
Rub or pulse cold butter into the dry mix until it resembles coarse crumbs.
Add the egg yolk and 1–2 tablespoons cold water; gently bring the dough together without overworking.
Form the dough into a disc, wrap and chill for 30 minutes.
Roll chilled dough to fit a 23 cm (9-inch) tart pan, press into the pan, trim excess and refrigerate for 15 minutes.
Line the shell with parchment and baking weights; blind bake at 180°C (350°F) for 15–18 minutes until set.
Remove weights and parchment and bake 3–5 minutes more to dry the base; cool on a rack.
Heat the cream until just simmering, then pour over the chopped dark chocolate and let sit 1–2 minutes.
Stir the chocolate and cream until smooth, then add the butter and corn syrup if using; mix until glossy.
Pour the ganache into the cooled tart shell, smooth the surface and chill for 2–3 hours (or overnight) until firm.
Garnish with a dusting of cocoa or flaky sea salt, warm a knife for clean slices, and serve.