Preheat oven to 170c or 338F
Melt your butter in the microwave for 30-60 seconds
Add sugar and vanilla and whisk well
Add the yogurt, egg and pumpkin puree and mix well
Add all dry ingredients and chopped chocolate or chocolate chips and using a spatula, fold the batter until all dry ingredients are well incorporated (do not over mix)
Add the batter to a lined brownie/cake pan (20cm x 20cm x 5cm)
Smooth out the batter and tap it on the bench a few times
Top with extra chopped white chocolate or chocolate chips
Bake at 170c or338F for 18-22 minutes
Let the cake cool down in the pan before cutting into 12 slices