**Prepare in Advance**: Make the dulce de leche a day ahead or well before you plan to serve the cake.

**Boil the Can**: Remove the label from the can of sweetened condensed milk. Place the unopened can in a large pot filled with water, ensuring the water level is at least 1 inch above the can.

**Simmer**: Bring the water to a simmer, then reduce the heat to low. Simmer for 1 ½ hours.

**Flip and Continue**: Use tongs to flip the can and simmer for another 1 ½ hours, adding water as needed to keep the can submerged.

**Cool Completely**: Remove the can with tongs and let it cool completely before opening. Stir the contents and set aside.

**Preheat Oven**: Preheat your oven to 350°F. Spray a 10-cup Bundt pan with non-stick spray.

**Cream Butter and Sugar**: In a large bowl, use an electric mixer to cream the softened butter and granulated sugar together until light and fluffy.

**Add Wet Ingredients**: Beat in the eggs and vanilla until pale and fluffy, about 1 minute. Mix in the pumpkin puree and buttermilk.

**Combine Dry Ingredients**: In a separate bowl, whisk together the flour, pumpkin pie spice, baking powder, baking soda, and salt.

**Mix Dry and Wet Ingredients**: Gradually add the dry ingredients to the wet mixture, mixing until fully combined.

**Bake**: Pour the batter into the prepared Bundt pan. Bake for 60-70 minutes, or until a toothpick inserted into the center comes out clean with just a few moist crumbs.

**Cool**: Let the cake cool in the pan for at least 30 minutes before turning it out onto a cooling rack to cool completely.

**Top with Dulce de Leche**: Once the cake is completely cool, pour the dulce de leche over the top and serve.