Preheat your oven to 350°F (175°C). In a large mixing bowl, cream together the unsalted butter, vegetable oil, granulated sugar, powdered sugar, and egg until light and fluffy, about 3-5 minutes.

In a separate bowl, sift together the all-purpose flour, cream of tartar, baking soda, and fine salt. Gradually add this dry mixture to the wet ingredients, stirring until just combined. Pour in the vanilla extract and mix well.

Chill the dough in the refrigerator for 10-15 minutes. Grease a heart-shaped baking pan with non-stick spray or butter. Press the dough evenly into the pan.

Bake for 16-18 minutes until edges are golden and a toothpick inserted in the center comes out clean. Let the cookie cool for about 5 minutes in the pan before inverting onto a wire rack.

In a mixing bowl, beat the room temperature unsalted butter until creamy. Add vanilla extract, raspberry preserves, and milk. Gradually mix in the powdered sugar until smooth and well-combined.

Once the cookie cake is completely cool, spread a layer of raspberry preserves over the top and frost it generously with your raspberry buttercream.