Cream butter and powdered sugar in a bowl of a standing mixer until light and fluffy.
Sift in flour, cocoa powder and salt.
Reduce the speed to low and beat in the flour mixture until just incorporated.
Gather dough and form a dish. Cover with plastic wrap and refrigerate for about 30 minutes to chill.
Preheat oven to 350F. Grease a 9-inch pie pan with a removable bottom.
On a lightly floured surface, roll out the dough to 1/4 inch thickness. Line the prepared pan with the rolled dough. Leave a tiny overhang of dough. Use a small kitchen knife to trim the excess and to score the tops of the tart with a fork.
Bake for 20 minutes. Let cool completely on a wire rack.
To prepare chocolate ganache filling, bring heavy cream almost to a boil in a small heavy sauce pan over medium heat.
Break chocolate into small pieces, and put them into another bowl. Pour in hot cream, let sit for a few minutes. Add corn syrup, and stir gently with a rubble spatula until smooth.
Pour ganache evenly over tart shell and spread with an offset spatula. Refrigerate overnight.
Leave tart at room temperature for an hour, garnish with raspberries or serve with a sprinkle of sea salt flakes.