Preheat the oven to 350°F/180°C/Gas 4.
In a medium bowl, combine the flour, sugar, cornstarch, and salt. Add melted butter, olive oil, and milk. Stir until all the ingredients are well-combined and a crumbly dough forms.
Press the dough into your tart pan, starting from the sides and then moving to the center. The sides should be a bit thicker than the bottom.
Prick the bottom of the tart shell with a fork and place in the oven to bake for 20-25 minutes until golden-brown and crisp.
Remove the tart shell from the oven and allow to cool for a few minutes.
Meanwhile, add the chocolate and heavy cream to a medium saucepan and place over low heat. Stir frequently while the chocolate melts to prevent burning.
Once the chocolate is melted, remove from heat and stir in the Kahlua and vanilla extract.
Pour the chocolate filling into the baked tart shell and place the tart in the refrigerator to set for two hours.
If using, melt white chocolate chis and drizzle over the set tart, or sprinkle with flaky sea salt.
Serve with whipped cream and/or fresh berries. Enjoy!