Preheat oven to 375F (180C). Coat 6 individual-sized ramekins with nonstick cooking spray; place on a baking tray.
Place butter, baking soda, salt, both sugars, eggs, vanilla, and flour in a food processor; pulse until just combined. Add in the prunes and water; pulse until nearly smooth.
Spoon the mixture into the ramekins, about 2/3 way full. Bake for about 18 minutes, until just firm to the touch and springs back to the touch. Remove from the oven and allow to cool slightly.
Preheat the broiler, putting the rack 4 inches (10 cm) from the heat source.
In the meantime, make the caramel sauce, place butter, cream, brown sugar, and salt in a medium saucepan over medium-high heat, whisking continuously until the mixture bubbles gently. Reduce heat to medium-low and cook until the sauce thickens and coats the back of a wooden spoon, stirring often, about 3-4 minutes.
Remove cakes from the ramekins and slice in half horizontally; place the bottom halves back in the ramekins and drizzle with a little of the caramel sauce. Then place the cake tops back on and drizzle with a little more of the caramel sauce. Place the ramekins under the broiler for about 1-2 minutes (watch it closely) until it bubbles and looks sticky.
Turn out cakes onto individual plates and drizzle with a little bit more of the caramel sauce, and a dollop of whipped cream or a drizzle of heavy cream.