Preheat oven to 350° and line a muffin tin with baking cups, parchment paper or with a non-stick spray.
In a small container combine the 1/4 cup of milk and the apple cider vinegar. Set aside for 5 minutes.
In a large bowl cream together the pumpkin puree, sugar, vanilla, softened butter and the remaining 1/4 cup of milk.
To the mixture add the flour, baking soda, baking powder, cinnamon and pumpkin spice. Stir until completely combined and no dry ingredients remain.
Pour the batter into each muffin tin until it just reaches the top. Set aside.
Prepare the oat crumble. To a bowl add the flour, sugar, cinnamon, melted butter and oats. Mix the a spoon or fork until a crumble mixture forms. Sprinkle a spoonful of the crumble on the top of each muffin.
Place the muffin tray in the oven and bake for 28-30 minutes. Remove from the oven and let the muffins cool.
If making the glaze, add all the ingredients in a small container and whisk until smooth. Add more powdered sugar or milk to reach your desired consistency. Drizzle the glaze over the muffins prior to serving.