Preheat your oven to 350°F (175°C) and grease three 8-inch round cake pans with butter or cooking spray, lining the bottoms with parchment paper.

In a medium bowl, whisk together the dry ingredients: flour, baking powder, baking soda, and salt.

In a large bowl, beat the butter and sugar until fluffy. Mix in the vegetable oil.

Add the eggs one at a time, mixing well after each, then add the vanilla extract.

Alternate adding the dry flour mixture and buttermilk, mixing until just combined.

Gently fold in the blueberries, then distribute the batter among the prepared cake pans.

Bake for approximately 30 minutes or until a toothpick comes out clean. Cool in pans for 10 minutes before transferring to a wire rack.

For the filling, cook blueberries, sugar, and lemon juice until bubbling. Thicken with cornstarch mixed with water.

For the frosting, beat the butter, then mix in cream cheese until smooth. Gradually add powdered sugar and blueberry puree.

Assemble the cake by layering with blueberry jam and frosting, smoothing over the entire cake before decorating.

Chill in the refrigerator for about 30 minutes before serving.