Prepare the butter by bringing it to room temperature. Sift the all-purpose flour and baking powder together. Preheat the oven to 180°C (356°F).

Cut the pumpkin into 2cm slices, remove the skin, and dice into 2cm cubes. Place the pumpkin and 1 tbsp of water in a microwave-safe container, cover loosely with plastic wrap, and microwave at 600W for 4 minutes. Drain and mash the pumpkin until smooth while still hot.

In a bowl, mix the butter until smooth. Add sugar and mix with a whisk until pale and fluffy. Gradually add the beaten egg in 3–4 portions, mixing well each time. Add milk and mix thoroughly.

Add the mashed pumpkin to the mixture and mix well. Fold in the sifted flour and baking powder using a spatula until no dry flour remains.