Preheat the oven to 350 F
Using a stand mixer or hand mixer or just a rubber spatula, mash the butter to make it smooth.
Add the sugar, mix it around to incorporate the sugar
Whisk the egg to the bowl
Next, whisk the sour cream to a smooth consistency
Sift all the dry ingredients (flour, baking powder, baking soda and salt) and this time don't whisk. Just fold using a rubber spatula or wooden spoon.
Next fold in the chopped walnut and rosemary.
Grease an 8-inch round pan and I like to also layer the bottom with a parchment paper.
Pour the cake batter to the pan.
Now for the strudel, mix the butter with sugar and flour and keep it crumbly.
Layer the top of the cake with the strudel.
I add some chopped walnut and rosemary on top.
Wrap the cake pan with wet baking strip or aluminum foil.
Bake the cake at preheated 350 F oven, in the middle rack for 45 minutes.
Let the baked cake cool down for a while before you slice and enjoy.