Preheat your oven to 350°F (175°C). Grease two 9-inch round cake pans with butter or non-stick spray, then line the bottoms with parchment paper. Set aside.

In a medium bowl, whisk together the flour, baking powder, baking soda, and salt.

In a large bowl, whisk melted butter, granulated sugar, and brown sugar until smooth. Add eggs, sour cream, Irish whiskey, and vanilla extract. Mix with an electric hand mixer on medium speed for 1-2 minutes until combined and slightly fluffy.

Gradually add the dry ingredients to the wet mixture, folding gently or mixing on low speed just until combined. Do not overmix.

Divide the batter evenly between the two prepared pans and smooth the tops. Bake for 30-35 minutes, rotating pans halfway through. Cake is done when a toothpick inserted in the center comes out with a few moist crumbs.

Cool cakes in pans for 10 minutes, then turn out onto wire racks to cool completely.

To make the buttercream frosting, beat softened butter with an electric mixer until creamy. Gradually add powdered sugar, alternating with Irish whiskey and heavy cream, beating until light and fluffy. Add vanilla extract and mix to combine. Adjust consistency with more cream or powdered sugar if needed.

Assemble the cake by placing one layer on a serving plate. Spread a thick layer of buttercream on top. Add the second layer and coat the top and sides with remaining frosting. Swirl frosting for a rustic look.