Preheat the oven at 170ºC/340ºF conventional oven and prepare a 20x20 cm /8x8 inch square pan with parchment paper.
Start by melting the butter over medium heat in a frying pan. Once melted, pour it into a medium sized mixing bowl and let it sit, while preparing the other ingredients.
80 g butter
In a bowl, sift together the all-purpose flour, baking powder, baking soda, pumpkin spice and salt. Set it aside.
180 g all-purpose flour, 1 ¼ tsp baking powder, ¼ tsp baking soda, ½ tsp salt, 2 ½ tsp pumpkin spice
Add the dark brown sugar to the bowl with the butter and with a hand whisk, mix it together. Then add the pumpkin puree, eggs and vanilla extract and mix it together until smooth and combined.
225 g dark brown sugar, 280 g pumpkin purée, 2 eggs, 1 tsp vanilla extract
Add the dry ingredients and stir it all together. Make sure there is no dry flour left in the batter.
Pour the cake batter into the prepared baking pan and spread the cake batter into an even layer. Then dollop the cinnamon sugar butter over the cake and with a knife, gently try and swirl it in. Bake it for 35-38 minutes or until a cake tester comes out clean.
Let the cake cool on a wire rack. After 30 minutes, gently remove the cake from the cake tin and let it cool down completely.