Heat oven to 325 F. Line eighteen 2 1/2-inch muffin cups with paper bake cups.
In a large mixing bowl, beat 1/2 cup, butter, granulated sugar and oil with electric mixer on medium speed to combine. Add egss one at a time, mixing well after each addition.
In a separate bowl whisk together flour, baking soda and salt; set aside.
Add half the dry ingredients to the butter mixture and blend well. Alternate between adding 1/4 cup sour cream and remaining dry ingredients, blending after each addition. Add 1 1/2 teaspoons of vanilla extract and blend.
Fill lined cups three-quarters full. Bake 20 minutes or until toothpcik inserted in center comes out clean.
Cool cupcakes on wire rack in pans 5 minutes. Remove from pans and cool completely on wire racks (20-30 minutes).
Pour 1/4 cup of chocolate liqueur in a shallow bowl. Dip tops of cupcakes in liqeuer. Frost cupckes with white frosting (recipe below) using a pastry bag with a star tip. You can also use a spoon to dollop frosting.
White Frosting: In a large mixing b owl beat 1/2 cup butter, 1/4 cup chocolate liqueur, 2 tablespoons whipping cream and 1 1/2 teaspoons pure vanilla extract with electric mixer on medium-high speed. Beat in one 16-ounce box powdered sugar until combined.