Prepare all ingredients beforehand to ensure smooth execution. Timing is crucial for caramel sauce, so have butter and cream ready at the right moment. Use a 2 1/2 to 3-quart heavy-bottomed saucepan since the sugar syrup will foam up and produce steam when butter and cream are added.

Combine sugar and water in a large saucepan. Boil over medium heat until it turns light to medium amber. Experience helps in perfecting the color, but it's better to be slightly light than too dark, which can taste burnt. Once the color is right, add butter quickly, stir until melted, then remove from heat and pour in cream, stirring constantly until smooth. Cool completely.

Grease two 8 or 9-inch cake pans and line with parchment paper. Preheat oven to 325°F.

Sift all-purpose flour, cake flour, baking powder, baking soda, and salt together and set aside.

In a mixer bowl, beat butter, sugar, and vanilla extract at high speed until light and fluffy. Slowly beat in vegetable oil, then eggs one at a time, mixing well after each addition.

Fold in dry ingredients alternately with milk, starting and ending with dry ingredients. Mix until just combined, then pour into prepared cake pans.

Bake for 30-35 minutes or until a toothpick comes out clean. Cool in pans for 10 minutes, then transfer to wire racks to cool completely.

For the frosting, beat all ingredients together until smooth and fluffy. Add milk a little at a time to reach the desired consistency, or more powdered sugar if too thin.

Cut cake layers in half to create 4 layers. Reserve 2/3 cup caramel sauce (1/2 cup for frosting and the rest to drizzle). Divide remaining caramel sauce into 3 parts to fill the inner layers. Frost with caramel frosting and drizzle with leftover caramel sauce.