Step 1: Line an 8×8 inch square baking pan with parchment paper, leaving some overhang on the sides for easy removal later.
Step 2: In a medium bowl, combine the crushed biscuits and the 150g of melted butter. Stir until the crumbs are evenly coated.
Step 3: Press the crumb mixture firmly into the bottom of the prepared pan using the back of a spoon. Place the pan in the refrigerator to chill while you make the caramel.
Step 4: In a medium saucepan, combine the sweetened condensed milk, 150g butter, brown sugar, and syrup. Melt over medium-low heat.
Step 5: Bring the mixture to a gentle boil, whisking constantly. Continue to boil for 5-8 minutes, or until the caramel thickens and turns a light amber color.
Step 6: Pour the hot caramel over the chilled biscuit base. Smooth it out with a spatula and return to the refrigerator for at least 1 hour until set.
Step 7: In a microwave-safe bowl, melt the chocolate chips and coconut oil in 30-second intervals, stirring until smooth.
Step 8: Pour the melted chocolate over the set caramel layer and spread evenly. Sprinkle with sea salt if desired.
Step 9: Chill the no bake millionaire shortbread bars for at least 2 hours, or until the chocolate is completely firm, before slicing.