Using an electric mixer, beat ¾ cup chilled mascarpone, 1 cup chilled heavy cream, and a pinch of kosher salt in a large bowl, starting on low speed and gradually increasing speed to medium-high as mixture thickens, until stiff peaks form. Cover mascarpone cream and chill until ready to serve.
Let remaining ¼ cup chilled heavy cream sit on counter to come to room temperature. Things will move quickly from here, so have all the remaining ingredients and a lighter or match at the ready.
Combine ¼ cup (packed; 50 g) light brown sugar, 3 Tbsp. banana liqueur or 2 Tbsp. water, 3 Tbsp. unsalted butter, cut into 1″ pieces, ¼ tsp. ground cinnamon, ⅛ tsp. ground nutmeg, and remaining ½ tsp. Diamond Crystal or ¼ tsp. Morton kosher salt in a 10″-diameter skillet. Set over medium heat. Cook, stirring, until butter is melted and brown sugar is dissolved, then bring to a boil and continue to cook, stirring often, until dark brown and thick, 8–10 minutes.
Arrange 3 large firm but ripe bananas, peeled, halved crosswise, then lengthwise, cut sides down, in pan, fitting in tightly, and increase heat to high. Cook, shaking pan occasionally, until bananas are browned in spots underneath, about 3 minutes (the brown sugar mixture pooling around the sides of the skillet might start to look separated and darkened in spots, but that’s okay—it will come back together when you add the cream later). Reduce heat to low and carefully turn bananas, cut side up. Add 3 Tbsp. dark rum, bourbon, rye, or cognac and, holding your head back, quickly ignite (be careful, the flames will reach about 2 feet high). Shake skillet until flames subside, then cook 2 minutes. Remove from heat.
Drizzle room-temperature cream around bananas, shaking and swirling skillet until cream mingles with brown sugar mixture and forms a smooth sauce.
Divide chilled mascarpone cream among plates. Place 2 banana pieces, cut side up, on each plate. Drizzle sauce remaining in pan over bananas. Top with ½ cup very coarsely chopped salted roasted macadamia nuts.