Place an oven rack in the center of the oven. Preheat the oven to 325 degrees F. Spray a 9-by-5-by-3-inch nonstick, metal loaf pan with vegetable oil cooking spray. Line the bottom of the pan with a 14-by-3 1/2-inch piece of parchment paper. Spray with vegetable oil cooking spray. Line the sides of the pan with a 14-by-7-inch piece of parchment paper. Set aside.
In a small, heavy saucepan, combine the caramel candies and heavy cream over low heat. Cook, stirring occasionally, until the candies have melted and the mixture is smooth, about 10 minutes. Set aside to cool slightly.
In a medium bowl, whisk together the flour, baking powder, and salt.
In a stand mixer fitted with the paddle attachment, beat the butter and sugar on high speed until light and fluffy, 2 to 3 minutes. Reduce the speed to medium and beat in the eggs and vanilla. With the machine running, gradually add the flour mixture in batches to form a thick batter. Beat in the peanut butter. Transfer half of the batter to the prepared pan. Pour 2 tablespoons of the caramel lengthwise along the center of the batter. Using the tip of a knife, swirl the caramel sauce into the batter. Place the remaining batter on top and pour another 2 tablespoons of the caramel sauce lengthwise along the center of the batter. Swirl the caramel sauce into the batter. Bake until a cake tester inserted into the center of the cake comes out with moist crumbs, 55 to 60 minutes. Cool for 10 minutes. Remove the cake from the pan and peel away the parchment paper. Cool completely on a wire rack, about 1 hour (center of cake will fall during cooling). Slice the pound cake and place on a serving platter. Warm the remaining caramel sauce over low heat. Drizzle the caramel sauce over the pound cake slices or serve alongside.