Prepare the pastry: whiz the flour, salt, powdered sugar and cold butter in a food processor in short pulses to a crumbly mixture. Add the egg yolks and process just until the pastry dough starts to form (avoid over-processing). Shape the dough into a ball, wrap in plastic wrap, flatten to a disc and refrigerate for 30 minutes.
Prepare the filling: Meanwhile, beat the egg yolks and honey in a bowl and set aside. Mix the cornstarch with a little heavy cream in a saucepan until smooth with no lumps. Add the remaining heavy cream while continuously whisking. Add sugar, salt, and ground cardamom and place the saucepan over medium-low heat. Cook, stirring continuously until thickened, about 10 minutes. Gradually and very slowly, pour this mixture into the bowl with egg yolks and honey, vigorously mixing. Mix until combined well into a smooth cream. Cover the surface of the cream with the plastic and cool in the refrigerator for at least 30 minutes.
Preheat the oven to 325°F and grease the tart pan with butter.
Roll the cold pastry between two sheets of parchment paper to ⅛ inch. Carefully transfer the piece of rolled out dough on top of the tart pan and use your hands to press the dough into the pan. Cut off any excess dough that hangs over the edge of the pan. Use any excess pieces of pastry to patch any tears. Freeze for 15 minutes.
Blind bake the dough: Remove the tart pan from the freezer and line it with parchment paper which has been cut into a circle about the diameter of the bottom of the pan. Line again with tin foil, making sure all of the crust is covered. Fill with pie weights or dried beans and then place into the oven for 10 minutes, until the crust has just slightly browned. Carefully remove the pie weights or beans, tin foil and parchment paper.
Pour the filling over the tart, smooth to an even layer and bake until the mixture is bubbling and amber in color, about 45 minutes. Cool for at least 2 hours.
To serve: sprinkle a pinch of flaky sea salt, fresh oregano blossoms or hyssop leaves on top of the tart.
