Preheat oven to 350° F. Butter an 8x8 pan or 8-inch round cake pan and line the bottom with parchment paper. The parchment is optional, but makes the cake easier to remove.
Whisk together egg yolks, sour cream, milk, and elderflower bourbon in a bowl and set aside.
In a separate bowl, whisk together flour, cornstarch, baking powder, baking soda, and salt.
Using a stand mixer with the paddle attachment or electric beaters, cream butter until broken down. Add sugar and elderflowers; cream until light and fluffy, about 2 minutes.
Add about one third of the flour mixture to the butter mixture and blend until almost incorporated. Then add about one third of the wet ingredients and fold in. Repeat alternating the dry and wet ingredients until all ingredients are just blended.
Pour the cake batter into the prepared pan and bake until lightly golden all over and center springs back when touched, about 30-35 minutes.
Place cake on a cooling rack and allow to cool completely.