Preheat: Preheat the oven to 350° F and line a 9x13 baking pan with parchment paper.
Cream butter and sugar: Cream the softened butter and brown sugar using either a stand mixer or a hand mixer until light and fluffy, 2-3 minutes.
1 cup softened unsalted butter, 1 cup dark brown sugar
Add in the rest of the ingredients: Add the all-purpose aflour, chocolate chips, instant coffee, baking powder, salt, vanilla extract (and optional espresso powder if you have it!) Mix until combined on medium speed. The mixture may seem crumbly, but should come together when you press it between your fingertips.
2 cups all-purpose flour, 1 cup milk chocolate chips, 2 Tbsp. instant coffee granules, 1/2 tsp. baking powder, 1/2 tsp. salt, 1/2 tsp. vanilla extract, 1/2 Tbsp. espresso powder
Press into pan: Press into a 9x13-inch oven-safe baking dish (you can line with parchment paper to make it easier to remove bars from the pan). Use clean hands to press the mixture down. Or use the back of a measuring cup or a rubber spatula to press down in an even layer.
Bake: Bake for 18-20 minutes.
Make the top layer: While the cookie layer is baking, heat the condensed milk and the butter in a small saucepan over low heat. Stir occasionally and heat on low until the condensed milk becomes nice and smooth, ~5 minutes. (Keep on low so it doesn't burn the bottom).
1 (14 oz.) can sweetened condensed milk, 2 tsp. vanilla extract, 2 Tbsp. unsalted butter
Add the vanilla: Remove from heat and stir in the vanilla extract.
Add the top layer: Once the cookie layer is done, pour the condensed milk on top in an even layer. Place back in oven and bake again for 8-10 minutes, or until the top layer looks set.
Let cool: Remove from the oven, it will be bubbling, and sprinkle with sea salt flakes. Let cool for at least 30 minutes-1 hour. The longer the better so the topping has time to set and not be goopy. You can place in the fridge or freezer to speed this process up.
2 tsp. flaky sea salt
Cut into diamond-shaped bars: Once the topping has cooled, cut diagonally across the bars beginning in one corner. Continue making cuts across the bars, moving from left to right. When you’ve cut all the way across, begin cutting from the corner above where you started.
Store and enjoy : Store in the fridge or freezer. We love these even more day 2 and 3...and 4, so they can easily be made in advance if needed.