In a large mixing bowl, combine the softened unsalted butter, light brown sugar, and granulated sugar. Using an electric mixer, cream the mixture on medium speed until it becomes light and fluffy, about 2-3 minutes.
Add the large egg and vanilla extract to the creamed mixture. Beat until well combined, ensuring everything is fully incorporated.
In a separate bowl, whisk together the old-fashioned oats, all-purpose flour, whole wheat flour, baking soda, and salt (if using). This step helps to evenly distribute the baking soda and prevent clumping.
Gradually add the dry ingredient mixture to the wet ingredients, mixing on low speed until just combined. Be careful not to overmix, as this can result in tougher cookies.
Using a spatula or wooden spoon, gently fold in the raisins and semi-sweet chocolate chips until evenly distributed throughout the dough.
Divide the dough into two equal portions. Roll each portion into a log, approximately 2 inches in diameter. Wrap each log tightly in plastic wrap and refrigerate for at least 2 hours or up to 24 hours.
When you're ready to bake, preheat your oven to 350°F (175°C). Line a cookie sheet with parchment paper to prevent sticking.
Remove the dough logs from the refrigerator. Use a sharp knife to slice the logs into 1/2-inch thick rounds. Place the rounds on the prepared cookie sheet, leaving space between each cookie as they will spread slightly during baking.
Bake the cookies in the preheated oven for 10-12 minutes, or until the edges are lightly golden. The centers may look slightly underbaked, but they will continue to firm up as they cool.
Once baked, remove the cookies from the oven and let them cool on the cookie sheet for 5 minutes before transferring them to a wire rack to cool completely.
