Place melted butter, maple syrup and sugar together in a large bowl. Whisk to combine.
Add in the eggs and whisk until smooth and well combined.
Add in the flour, oats, salt, spices, bicarb and baking powder and stir with a wooden spoon/spatula until the mixture starts to come together.
Add in the pumpkin purée and mix until a soft dough forms.
Cover the bowl with cling film and place in the fridge for 30 minutes to chill.
Once chilled, preheat the oven to 180C/350F and line 2 baking trays with grease proof paper/a Silpat/silicone mat.
Using a 1.5tbsp cookie scoop, place balls of dough 2 inches apart and flatten them a little bit with the palm of your hand.
Place in the oven for 9-11 minutes until the edge are lightly golden and the cookies look set. They will also be quite matte, with a soft sheen if you look in the light of the oven. If they are still wet and shiny, give them a couple more minutes.
Leave to cool completely on the baking trays. At this stage you can either leave them as is, or add the glaze.
Once cool, make the glaze if required. Place icing sugar and cinnamon into a medium sized bowl and stir briefly. Add in the maple syrup and beat until the mixture comes together.
Add in the milk 1/2tbsp at a time until you have a thick, gloopy consistency which will stay on the top of your cookies and not run off down the sides. When you pick up a spoon and tilt it, you should see "ribbons" as it falls back into the bowl.
Place kitchen paper underneath a wire rack and transfer all the cookies to the rack. Spoon 1/2tsp of the glaze into the centre of each cookie and spread with the back of your spoon.
It should roll over the edges ever so slightly, but most of the icing should stay on the top. Sprinkle a little cinnamon on top of the icing and leave to set for 15 minutes.
Once the icing is set, devour! Pumpkin Oatmeal Cookies will keep in an airtight container at room temperature for 2 days with the glaze and 5 days without. You can also keep them in the fridge for up to 4 days with the glaze and a week without.