Beat the softened butter at medium speed with an electric mixer for 3 minutes until pale yellow and creamy. Gradually add the white sugar and the brown sugar to the butter at medium speed for 7 minutes.

In a medium-sized bowl, sift the twice sifted Swans Down cake flour with the baking soda. Slowly add the sifted flour to the butter mixture 1 cup at a time mixing at medium speed.

Note: The batter will be thick like sugar cookie dough.

Add the room temperature egg yolks to the batter one at a time and mix until combined.

Set the mixer to low speed and add the sour cream and the vanilla. Stop mixer the minute it is just combined. Avoid over mixing.

Note: The batter will loosen with this step.

Using a hand-held mixer in a separate bowl, beat the egg whites until stiff peaks form. Gently fold the egg whites into the batter.

Pour the batter into the prepared pan carefully.

Bake for 2 hours or until a knife comes out clean. Cool in pan on wire rack for 15 minutes. Remove from pan and cool completely on wire rack.

In a saucepan combine 1/2 cup BSB (Brown Sugar Bourbon) with 1/2 cup brown sugar and cook at medium heat until the sugar dissolves. Remove from the heat.

Spoon the Brown Sugar Bourbon Syrup over the cake.