Preheat your oven to 300°F. Grease the sides of a 9x13 inch cake pan.

In a large mixing bowl, combine all the dry ingredients: all-purpose flour, granulated sugar, natural unsweetened cocoa powder, baking soda, and salt. Mix until well combined.

Add the eggs, milk, vegetable oil, 0.5 cup of Baileys Irish Cream, and 1 teaspoon of vanilla extract to the dry ingredients. Mix on medium speed until well combined and smooth.

Slowly and carefully pour the boiling water into the batter while mixing. Be cautious of splashing. Mix until the batter is smooth.

Pour the prepared batter into the greased 9x13 inch cake pan. Bake for 40-45 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.

Once the cake is removed from the oven, immediately poke holes all over the top. A knife sharpening rod or a thick straw works well for this.

While the cake is still warm, place the semi-sweet chocolate chips in a medium-sized heatproof bowl. In a microwave-safe measuring cup, combine the sweetened condensed milk and the remaining 0.5 cup of Baileys Irish Cream. Heat until the mixture begins to boil, then immediately pour over the chocolate chips.

Whisk the chocolate mixture until smooth and the chocolate chips are fully melted. Immediately pour this warm mixture over the cake, spreading it evenly and ensuring it seeps into all the poked holes. Do not let the mixture sit before pouring, as it will thicken and not soak into the cake as effectively.

Transfer the cake to the refrigerator to cool completely and allow the soak to fully absorb, about 1-2 hours.

Once the cake has completely cooled, prepare the whipped cream topping. In a large mixer bowl, combine the cold heavy whipping cream, powdered sugar, 0.25 cup natural unsweetened cocoa powder, 2 tablespoons Baileys Irish Cream (if using), and 0.5 teaspoon vanilla extract. Whip on high speed until stiff peaks form.

Spread the Baileys whipped cream evenly over the cooled cake. Garnish with mini chocolate chips and a drizzle of chocolate sauce, if desired. Refrigerate the cake until you are ready to serve.