Line an 8x8 inch baking pan with parchment paper, leaving an overhang for easy removal. Lightly grease the parchment.

In a heavy-bottomed saucepan, combine butter, sugar, and corn syrup. Stir constantly over medium heat until butter is melted and sugar dissolved.

Bring to a boil without stirring. Cook, swirling occasionally, until a candy thermometer reads 300°F (150°C). This takes about 10-15 minutes.

Remove from heat and slowly pour in heavy cream; it will bubble vigorously. Stir constantly until smooth. Stir in vanilla and salt.

Pour toffee into the prepared pan and spread evenly. Sprinkle with chocolate chips (optional) while still warm, letting them melt slightly before spreading.

Let cool completely at room temperature for at least 2 hours, or preferably overnight, to harden.

Lift toffee from pan using parchment paper overhang. Break into pieces and sprinkle with sea salt flakes (optional). Store in an airtight container.