In a medium bowl, combine the flour, quick oats, baking powder, salt, and cinnamon. Set aside.
In a standing mixer, cream brown sugar, sugar, butter, and vanilla extract until pale, about 3-5 minutes. Add eggs one at a time mixing until combined. Add dry ingredients and mix just until combined. Dissolve the baking soda and hot water, then add to the bowl and mix on medium speed for 1 minute.
Using a small cookie scoop or 2 tablespoons of dough onto parchment-lined baking sheets at least 2 inches apart. Bake for 9-11 minutes just until the cookies are no longer shiny. Let the cookies come to room temperature while making the filling.
For the filling:
In a small saucepan over medium heat, whisk flour, milk and cherry liquid together, stirring constantly, until thickened into a paste. Remove from heat, cover and let come to room temperature.
Cream the butter and sugar together until fluffy and white, about 5-10 minutes. Add in the cooled paste mixture and continue to whip for another 3-5 minutes or until a light, fluffy cream appears. Add in the cherries and mix.
Using a piping bag or spoon, spread about 2 tablespoons of the filling onto the bottom of a cookie, sandwich the second cookie on top and press to adhere. Enjoy!