1Mix the vinegar with milk and let it sit for 30 minutes to create buttermilk.
2In a large bowl, beat eggs, sugar, and vanilla with an electric mixer for 1 minute. Add the buttermilk and oil, and mix for another 30 seconds.
3Sift together flour, baking powder, baking soda, and salt. Gradually fold the dry ingredients into the wet mixture with a hand whisk until just combined.
4For the streusel, combine walnuts, flour, melted butter, brown sugar, salt, and cinnamon in a separate bowl and mix well.
5Pour half the cake batter into a greased baking pan. Add half the apples and half the streusel on top. Repeat with the remaining batter, apples, and streusel.
6Bake in a preheated oven at 180°C (350°F) for 40 minutes, then reduce to 170°C (340°F) for the final 10 minutes. Bake for a total of 50 minutes.
7Let the cake cool for 10 minutes before removing it from the pan. Place on a cooling rack and allow to cool completely before slicing.
8Optional: Drizzle with caramel sauce before serving.