Preheat oven to 325℉.
Grease and flour a bundt pan.
Peel, core, and chop apples. Toss in a bowl with lemon juice and set aside.
Combine oil, sugar, and vanilla in a bowl. Mix using a hand mixer.
Add eggs and vanilla yogurt and mix again.
In a separate bowl, sift flour. Add salt and baking soda.
Add dry ingredients to wet, a little at a time.
Once the batter is smooth, fold in apples and walnuts.
Pour into greased and floured bundt pan. Bake for 55-60 minutes or until a fork inserted comes out clean.
Cool.
Turn upside down onto a plate.
Melt butter in a small pot over medium-low heat.
Add brown sugar. Stir until melted.
Add milk.
As it starts to boil, add cinnamon, a little at a time, and stir.
Cook for 30 seconds and stir in vanilla.
Set aside and allow to cool for 1 minute.
Pour over cake.