In the bowl of a stand mixer fitted with the paddle attachment (or a large mixing bowl with an electric hand mixer), combine the softened butter, packed light brown sugar, egg, and vanilla; beat on medium-high speed until creamed and well combined, about 4 minutes.
Stop and scrape down the sides of the bowl. Add the all-purpose flour, old-fashioned rolled oats, cornstarch, baking soda, and the optional pinch of salt; beat on low speed until just combined, about 1 minute.
Scrape the bowl again and add the diced protein bars, dried cranberries, pumpkin seeds (or substitute), and raisins; beat on low speed just until evenly distributed, about 30 seconds.
Using a large cookie scoop, a 1/4-cup measure, or your hands, form approximately 12 equal mounds of dough. Roll each into a ball and flatten slightly. If desired, press additional dried fruit or protein bar pieces onto the tops so no large pieces sit directly on the baking sheet.
Place the dough mounds on a large plate or tray, cover with plastic wrap, and refrigerate for at least 2 hours (or up to 5 days) to prevent excess spreading during baking.
Preheat the oven to 350°F (175°C). Line a baking sheet with a Silpat or parchment paper (or spray lightly with cooking spray). Arrange chilled dough mounds at least 2 inches apart (about 8 per sheet).
Bake for about 11 minutes, until the edges have set and the centers are just set and still slightly glossy; do not overbake. Let cookies cool on the baking sheet for about 10 minutes so they firm up before serving.
