Preheat oven to 350 degrees F. Lightly grease cupcake/muffin tins or line with cupcake liners.
In a medium sized bowl, use a hand mixer to beat the cream cheese, sugar, egg, Irish cream liquor and salt until smooth. Set aside about ¼ cup of the chocolate chunks for later. Stir in the remaining chocolate chunks and set aside.
In a small bowl, combine the eggs, sour cream, and vanilla extract; set aside.
In a large saucepan over medium-low heat, add the Guinness beer and butter. Heat just until the butter is melted; remove from heat.
Off the heat, whisk in the sugar and cocoa powder into the melted butter mixture until blended. Add the prepared egg mixture and whisk again until the eggs are well combined. Add the flour and baking soda and whisk together until smooth.
Divide the Guinness cake batter equally between the muffin tins, filling each approximately 1/2 full. Next add a large heaping tablespoon of the Irish Cream Cheese filling in the center of the Guinness batter. (Muffin tins should end up no more than 2/3 full)
*Note: If you would like a marbled effect instead: Use a knife or toothpick to gently swirl the Irish Cream filling and Guinness together.
Sprinkle the tops with the reserved chocolate chunks.
Place filled muffin tins on middle and bottom rack of oven. Bake for approximately 10 minutes and then rotate the pans between oven racks. Bake for another 12 to 13 minutes until the cupcakes have risen and the internal temperature registers approximately 205 to 209 degrees F. on your cooking thermometer. If you do not have a cooking thermometer, you can insert a tooth pick into the cupcake and pull out. If the toothpick comes out clean then cupcakes are done. Cooking for additional one minute increments if needed until cupcakes are done
Remove from oven and let cupcakes completely cool on a wire rack before turning out.
Makes 12 large muffins or 30 cupcakes.