Measure out all ingredients for the custard and position within reach of stove. Combine the egg, egg yolks and cornstarch in a minimum 2 cup heat safe measuring cup until smooth.
Make the caramel. Over medium high heat, heat the brown sugar and water until bubbly, then cook about 5-7 minutes more. Watch closely to ensure the mixture does not burn. You'll smell a very caramelly smell when it's time to move to the next step.
Slowly and carefully add the milk and cream while whisking constantly until the caramel has dissolved into the milk and cream. Add the salt and bring just to a boil and turn the heat down to medium low.
Temper the egg and cornstarch mixture by slowly ladling the hot milk and cream mixture into the heat safe measuring cup while stirring constantly until nearly full. Keep the mixture moving while adding the hot milk and cream to ensure the eggs do not scramble.
Slowly and while constantly whisking, pour the tempered egg and cornstarch mixture into the hot milk and cream. The custard will begin to thicken almost immediately. Continue to whisk until the custard as thickened enough to jiggle when you give the pan a little shake.
Remove the custard from the heat and stir in the bourbon (or vanilla) and the butter until smooth. Portion into ramekins, small mason jars, martini glasses or similar and chill for around 3 hours.
Immediately before serving, in a 2 cup heat proof measuring cup, melt the chocolate chips and coconut oil in the microwave, stirring every 30 seconds until completely melted and pour over the top of the chilled custards. Sprinkle with flakey sea salt and serve.