In a large bowl, combine the pumpkin puree, sugar, egg yolk, coconut oil, vanilla and pumpkin pie spice. Whisk together until smooth.
Add in the oat flour, baking soda, baking powder, lemon juice, and salt and stir together to combine- the batter may be slightly soft
Chill the dough for 40 minutes.
Preheat the oven to 350 and prepare a baking sheet with parchment paper
In a small bowl, combine the brown sugar and cinnamon for the coating.
Remove the dough from the fridge once chilled, and scoop out 2 tbsp worth of dough. Roll each ball in the cinnamon sugar mix and place it onto the baking sheet, repeat with remaining dough. Make sure not to overcrowd the pan. I typically bake these on two sheet pans or in batches.
Bake the cookies for 10-12 minutes. Once you remove them from the oven, you can flip a large mug or ramekin over each cookie while they're still on the pan and gently swirl them around 1-2 times to round out any uneven edges.
Let them cool for at least 10-15 minutes before removing from the pan.
Keep stored in an airtight container for up to 5 days.