Coat the inside of a 10 inch cast iron skillet with butter. Set it aside for now.
Combine the flour, baking powder, and salt in a small bowl. Set it aside.
In your mixing bowl, beat the sugar and butter together until fluffy. Add the egg yolks and mix in.
Add half of the dry ingredients and mix in.
Mix in the milk and then the remaining dry ingredients.
In another bowl, whisk the eggs until stiff peaks form. I use an immersion blender with whisk attachment to make this easy. Fold the egg whites and vanilla extract into the batter.
Spread half of the batter into your prepared pan.
In a bowl, combine the filling ingredients and stir well.
Spread the filling over the cake batter in the pan.
Spoon the remaining batter all over the top of the filling.
Bake for 30-35 minutes, or until the cake tests done with a wooden pick in the center.
Transfer the pan to a cooling rack.
Prepare your glaze if you wish. Put the powdered sugar in a small bowl.
Add the vanilla extract and just enough milk to make a glaze that is thin enough to drizzle.
Drizzle the glaze all over the top.
Allow the cake to cool and then enjoy!
Video
Notes
I prepared this cake in a 10 inch cast iron skillet. If you use a baking pan that is of a different size, you may need to adjust the baking time.This cake can also be made with pecans instead of walnuts as well if you like.