Pumpkin Streusel Muffin

1/2 cup sugar pumpkin purée

1/2 cup whole milk

1/3 cup vegetable oil

1/2 cup sugar

1 egg beaten

1 3/4 cups flour

2 tbsp baking powder

1/2 tsp salt

1 tsp cinnamon

1/4 cup brown sugar

1/2 tsp cinnamon

2 tbsp butter

1/3 cup walnuts chopped