Step 1: Preheat your oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan.
Step 2: In a large bowl, combine the cake mix, buttermilk, vegetable oil, eggs, white vinegar, and 1 teaspoon vanilla extract. Beat with an electric mixer on medium speed for 2 minutes, until smooth.
Step 3: Pour the batter into the prepared baking pan and spread evenly.
Step 4: Bake for 30-35 minutes, or until a wooden skewer inserted into the center comes out clean. Let the cake cool in the pan for 15 minutes.
Step 5: While the cake cools, combine the sweetened condensed milk and 1/2 cup whole milk in a small bowl. Whisk until well combined.
Step 6: Using the back of a wooden spoon or a skewer, poke holes evenly over the top of the warm cake, about 1 inch apart. Pour the condensed milk mixture slowly over the cake, ensuring it seeps into the holes.
Step 7: Refrigerate the cake for at least 30 minutes to allow the liquid to soak in completely.
Step 8: Prepare the cream cheese frosting: In a large bowl, beat the softened cream cheese and butter with an electric mixer until light and fluffy. Gradually add the sifted powdered sugar, beating until smooth. Stir in 1 teaspoon vanilla extract and a pinch of salt.
Step 9: Once the cake is chilled, spread the cream cheese frosting evenly over the top. Garnish with fresh cranberries, rosemary sprigs, or festive sprinkles if desired.
Step 10: Refrigerate for at least 1 hour before serving to allow the frosting to set. Slice and serve chilled.