Position a rack in the center of the oven and preheat to 350 degrees Fahrenheit. Coat a 9 × 13-inch cake pan (see Note) with nonstick spray, then line it with Gefen Parchment Paper.

In a medium bowl, whisk together the flour, baking powder, and salt.

Crack the eggs into a large bowl. Add the sweetened condensed milk and the sugar and, using a hand mixer, beat on medium speed until combined, about one minute. Reduce the speed to low and add half the flour mixture, mixing until just combined. Add the oil, then the remaining flour mixture, and mix until just combined.

Scrape the batter into the prepared pan. Smooth out the top with a spatula.

Bake, rotating the pan halfway through, until golden and a tester inserted into the center comes out clean, 25 to 30 minutes. Let the cake cool for 15 minutes in the pan, then turn it out onto a wire rack to cool completely, about two hours. The cooled cake can be tightly wrapped and stored at room temperature for up to three days or in the refrigerator for up to five days.

Poke the cake all over with a fork, making sure to get all the way to the bottom.

Split the cake in half horizontally (like a layer cake). Place half the cake, cut-side up, in a clean 9 × 13-inch cake pan or another three or four-quart baking dish (if the dish isn’t rectangular, just break pieces of cake to fit in an even layer). You may need to trim the edges of your cake so that it fits flat in the bottom of your dish.

In a small bowl, stir together the coffee, condensed milk, and rum (if using). Pour about half the mixture over the cake, or as much as you need to cover it completely. Let soak for at least 10 minutes. If you notice parts that seem a little dry, poke more holes in the cake at this point and add a little more liquid.

Meanwhile, in a large bowl, combine the cream, mascarpone, sugar, vanilla, and salt. Using a hand mixer, beat on medium speed until the cream holds medium peaks, one to two minutes.

Spread half the whipped cream mixture over the top of the soaked cake. Place the second half of the cake, cut-side up, on top of the whipped cream, then pour over more coffee mixture to completely soak the cake. (You may have more liquid than you need—do not pour so much that you create a moat around the cake. You could use what’s left to make a side cake with your cut-off cake edges!) Top with the remaining whipped cream.

Chill the assembled cake in the refrigerator for at least 6 hours, or ideally 24 hours, before serving. (It will keep, covered, in the fridge for up to four days.) In a small bowl, stir together the cocoa powder and cinnamon, then use a fine-mesh sieve to dust the chilled cake with the mixture.