3/4 cup plus 1 tablespoons sweetened condensed milk
1 1/2 teaspoon pure vanilla extract
1 1/2 tablespoons dark rum
1 1/3 cup heavy cream
Layer the bottom of a 9-inch square baking pan with crackers. Count how many you have and multiply by 2. Once you figure out how many crackers you need, brush the crackers on both sides with the melted butter.
To make the brownies, place racks in the middle and lower positions of the oven and preheat the oven to 350 degrees F. Lightly grease the baking pan and line with parchment paper, leaving extra parchment hanging over the sides. Have an extra baking sheet ready too.
In a big bowl, melt the chocolate chips and butter in the microwave in 30-second increments, stirring between each increment until fully melted. Whisk in both sugars, the eggs, ground espresso, and vanilla until smooth and slightly fluffy. Sift in the flour, cocoa powder, and salt. Fold the ingredients together with a spatula or whisk until smooth.
Pour the brownie batter into the prepared pan and smooth the top. Arrange half of the crackers on top of the batter. If a whole cracker doesn’t fit at the edge, just cut a cracker with a sharp knife to fit. Once you have completely covered the brownie batter with crackers, scatter the rest of the crackers on the baking sheet.
Place the brownie-cracker on the oven’s middle rack and the crackers on the lower rack. Bake until the brownie-cracker is slightly underdone, about 25 minutes. A wooden skewer inserted into the center should come out moist with wet crumbs. Take it out of the oven and take the crackers on the baking sheet as well.
Let the brownie-cracker cool for 10 minutes, then transfer the pan to the freezer to cool for 30 minutes. The brownie-cracker should be completely cool before you continue. (This will keep, tightly wrapped in plastic in the freezer for up to 1 week.)
Transfer the brownie-cracker to a work surface. Line the pan with another piece of parchment. Invert the brownie-cracker and remove the first layer of parchment. Place back in the pan, cracker layer facing down. Take note of where the uneven crackers are so you can match that side on top. Return to the freezer while you make the ice cream.
To make the ice cream, in a small bowl combine the condensed milk, vanilla, and rum.
In the bowl of a stand mixer fitted with a whisk attachment (a handheld electric mixer will also work), whisk the heavy cream on medium high until soft peaks form, about 3 minutes. Gently fold the condensed milk mixture into the whipped cream until just combined.
Pour the ice cream on top of the brownie cracker and smoothe the top with a spoon. Arrange the remaining crackers on top, making sure to arrange them so they match the bottom layer for easy cutting. Return the pan to the freezer to harden for at least 8 hours, though overnight is best.
When ready to slice, cut between the crackers to form little sandwiches. Wrap each mini sandwich with plastic wrap and freeze for up to two weeks. Or serve to many and enjoy with sticky fingers.