Preheat your oven to 350°F (175°C).

In a large mixing bowl, combine softened unsalted butter with granulated sugar and powdered sugar. Beat on medium speed until light and fluffy, about 3-4 minutes.

Mix in vegetable oil, egg, and vanilla extract until smooth, about 1-2 minutes.

Whisk together flour, salt, baking soda, and cream of tartar in a separate bowl. Gradually add to the wet mixture, mixing on low speed until just incorporated.

Scoop the cookie dough onto a lined baking sheet, flatten slightly, and roll in granulated sugar.

Bake for approximately 12 minutes, or until the edges are set but not browned. Let cool on the sheet for a few minutes, then transfer to a wire rack.

Beat together cream cheese, granulated sugar, and vanilla extract until smooth and creamy, about 2-3 minutes.

Frost each cooled cookie generously with the cream cheese mixture.

Arrange fresh strawberries, raspberries, and blueberries on top of each frosted cookie.

Chill the assembled tarts in the refrigerator for at least 30 minutes before serving.