Preheat oven to 350F. Line two large baking sheets with parchment paper.
In a medium size bowl add flour, cornstarch, baking soda, cinnamon and salt. Whisk to combine.
In a large bowl, add butter, brown sugar and granulated sugar. Using an electric mixer, beat on medium speed for one minute. Scrape down the sides of the bowl with a spatula.
Add eggs and vanilla. Beat with an electric mixture on medium until dough is smooth. Use a spatula to scrape down the sides of the bowl.
Add half of the dry mixture to the dough. Beat with an electric mixer on low speed until the flour starts to incorporate. As the flour mixture starts to incorporate, you can increase the speed of the electric mixer to medium. Use a spatula to scrape down the sides of the bowl as necessary. Repeat with remaining dry mixture.
Add oatmeal and raisins. Use a spatula to fold in the oatmeal and raisins.
Using a large spoon or your hands, shape approximately 2 tablespoons of dough into a ball. Place dough ball onto baking pan. Repeat until the baking sheet is full. Leave 1.5 - 2 inches between cookies so they have room to spread when baking.
Bake cookies for 10-12 minutes or until the tops start to turn golden brown. Remove from the oven. Let cookies cool slightly before transferring to a wire rack to cool completely. Makes 24 cookies.