Preheat your oven to 350°F (175°C) and line a 9-inch round cake pan with parchment paper. Grease the sides with butter.

In a large mixing bowl, combine 1 cup of pumpkin puree, 1 cup of brown sugar, 2 large eggs, and 1 teaspoon of vanilla extract; blend until smooth.

In a separate bowl, whisk together 1 ½ cups of all-purpose flour, 1 teaspoon of pumpkin pie spice, 1 teaspoon of baking powder, ½ teaspoon of baking soda, and ½ teaspoon of salt.

Gradually fold the dry ingredients into the pumpkin mixture until just combined; pour into the prepared cake pan.

Bake for about 25 minutes, or until a toothpick inserted in the center comes out clean; allow to cool for 10 minutes.

In a saucepan, heat 2 cups of heavy cream over medium heat until steaming; whisk together ½ cup of granulated sugar and 4 egg yolks until pale.

Gradually add the hot cream to the egg mixture, whisking constantly to prevent curdling; strain the mixture into a clean bowl.

Pour the custard over the cooled cake, then prepare a water bath by placing the cake pan in a larger roasting pan filled with hot water.

Bake at 300°F (150°C) for 35-40 minutes until the custard is set around the edges; cool for 15 minutes then chill in the fridge for 4 hours or overnight.

Mix together ½ cup of brown sugar, ½ cup of all-purpose flour, ¼ cup of softened unsalted butter, and a pinch of salt for the streusel topping; bake until golden.

Top the chilled cake with whipped cream, then the baked streusel, and drizzle with caramel sauce before serving.