2. In a large bowl, cream together the butter and sugar until well combined.

3. Add the egg yolks, whiskey, and vanilla extract to the butter mixture, mixing until thoroughly blended.

4. Gradually incorporate the sifted all-purpose flour along with the baking powder and salt, alternating with the milk. Mix until smooth.

5. In a separate bowl, beat the egg whites until stiff peaks form. Gently fold them into the batter.

6. Pour the batter into the prepared cake pan.

7. Bake in the preheated oven for 1 hour, or until a toothpick inserted into the center comes out clean.

8. Unmold the cake while still warm.

9. Enjoy your butter cake.