Combine flour, oats, cinnamon, baking soda, salt, and cornstarch. Mix well and set aside.
In a large bowl cream together butter and both sugars until well combined.
Add eggs and molasses and beat until fluffy.
Add the dry ingredients a little at a time until just fully incorporated. Do not over-mix.
Add raisins or chocolate chips(if using).
Cover cookie sheets in parchment paper(don't skip this step or your cookies may be flat!)
Roll large balls of cookie dough(4 tablespoons each) and place on cookie sheet. Place no more than 4-6 cookies on a sheet. They will spread when baking.
Let cookie dough balls rest in the fridge for at least an hour or longer. Don't skip this step! It's important.
Preheat oven to 375 degrees.
Bake cookies for 12-13 minutes or until the edges are golden brown. If the cookie is still a little round when they are done, just press it down a bit with the back of a spoon. It's ok for the cookies to be a bit doughy when you first pull them out of the oven. They will continue to bake a bit on the pan as they are cooling. Move them to a cooling rack to finish cooling once they've set up completely.
Allow cookies to cool completely before assembling the ice cream cookie sandwiches.