Preparation Steps

• Preheat the oven to 325°F. Spread the flour in an even layer on a rimmed baking sheet and bake for 7–8 minutes to heat-treat; let it cool completely before using.
• Line an 8-inch square pan with parchment paper, leaving a couple inches of overhang on two sides to make lifting the bars easier.
• In a large bowl, beat the softened butter and brown sugar together (by hand with a sturdy spoon or with a mixer) until the mixture is pale and slightly fluffy, about 2–3 minutes. Stir in the vanilla.
• Whisk the cooled, heat-treated flour together with the cinnamon and salt. Add about half the flour to the butter mixture and fold to incorporate, then pour in half of the sweetened condensed milk and mix on low. Add the remaining flour, then the rest of the condensed milk, mixing until a thick, cohesive dough forms (avoid overworking).
• Fold the mini chocolate chips into the dough, scraping the bowl so everything is evenly distributed. Transfer the dough to the prepared pan and press it firmly and evenly into the pan using the back of a spoon, a piece of parchment, or lightly dampened hands.
• Chill the pressed dough in the refrigerator for about 90 minutes, or until it is firm to the touch.
• While the base chills, set up a double boiler: simmer an inch of water in a small saucepan and place a heatproof bowl over it (off the heat). Add the chocolate chips and butter to the bowl and stir gently until completely melted and smooth.
• Pour the melted chocolate over the chilled cookie dough, spreading into an even layer. If desired, sprinkle a small pinch of flaky sea salt over the top.
• Return the pan to the fridge and chill for another 30–45 minutes, or until the chocolate is set. Lift the slab from the pan using the parchment overhang and slice into 16 bars. Store in an airtight container in the refrigerator for up to 5 days.