Preheat oven to 300°F. Line a cookie sheet with parchment paper and spread the flour evenly over the parchment paper. Let the flour cook for about 5-6 minutes and then remove from the oven. Allow the flour to cool completely.
Cream the butter, sugar, and sweetened condensed milk for 2-3 minutes using an electric mixer. Add the vanilla extract and salt, mix for another minute. Add the flour and 2 cups of chocolate chips, mix again until well combined.
Transfer the cookie dough to a 9x9 pan lined with parchment paper. Press it into the pan evenly. Place the pan in freezer to chill in the meantime.
Microwave the remaining 1 cup of chocolate chips with ¼ cup heavy cream and 1 TBSP butter for 1:30. Stir gently until the chocolate melts and the mixture is smooth. Let it cool for 5-10 minutes, stirring often.
Remove the pan from the freezer, pour the chocolate ganache on top and spread it quickly to coat the cookie dough.
Microwave the white chocolate chips for about 45 seconds. Stir until smooth, returning to microwave for :15 increments if needed. Drizzle lines on top of chocolate ganache. Top with coarse sea salt, if desired.
Cover and chill for at least 4 hours. Cut into bars and enjoy!
Notes
Store leftovers in an airtight container in the fridge for up to a week. Bars can be frozen for up to a month.