Preheat Oven: Set to 350°F.
Cream Butter and Sugar: Beat butter with an electric mixer for 2 minutes until fluffy. Add sugar and vegetable oil, and beat for another 4 minutes until light and fluffy.
Add Eggs and Vanilla: Mix in eggs one at a time, ensuring each is fully incorporated before adding the next. Add vanilla extract.
Combine Dry Ingredients: Add flour, baking powder, and salt to the wet mixture. Mix until just combined.
Form Cookies: Scoop the dough into balls using a large cookie scoop (about 1/3 cup per ball) or smaller if preferred. I made 10 large cookies, each about 4 inches in diameter.
Prepare Baking Tray: Place dough balls on a tray lined with parchment paper or a silicone mat. Press down slightly to flatten.
Bake: Bake for 15 to 18 minutes until set on top and lightly golden around the edges. They should be firm but not overly brown.
Cool: Remove from oven and let cool completely before frosting.