How to Make It
Preheat oven to 350°. Grease a 9-inch square metal cake pan with butter. In a small heatproof bowl, pour the boiling water over the dates; stir in the baking soda. In a medium bowl, whisk together the flour, baking powder, and salt.
In the bowl of a stand mixer fitted with the paddle, beat 1/4 cup of the butter with 3/4 cup of the brown sugar at medium speed until light and fluffy, about 4 minutes. Beat in the eggs, one at a time. At low speed, beat in the flour mixture until just combined. Add the date mixture and bananas, and beat at low speed until just combined.
Scrape the batter into the prepared pan, and bake until a toothpick inserted in the center comes out clean, about 30 minutes.
Meanwhile, in a small saucepan, combine the heavy cream, the remaining 1/4 cup of butter, and the remaining 1 cup of brown sugar. Bring to a gentle boil over moderate heat, and cook until slightly thickened and deep golden, about 3 minutes. Keep warm.
Transfer the pan to a wire rack. Using a skewer or toothpick, poke holes all over the cake. Pour half of the warm sauce over the cake, and let stand until absorbed, about 10 minutes. Serve warm with the remaining sauce and the whipped cream.