First prepare the caramel sauce by combining the sugar, vinegar & water in a medium heavy bottom sauce pot stirring only until has dissolved not after it starts to boil or you will get crystals in your caramel! The acid helps to prevent this, but it can still happen if you stir it after it starts boiling
Watch it carefully as it turns from a light to medium amber, take it off the heat when it is medium amber as it will continue to cook & get darker after you remove it from the heat
Slowly & carefully pour in the room temperature plant milk & then add the vegan butter & salt stirring slowly & carefully. Once the bubbling subsides stir it to a smooth sauce
Divide this caramel sauce into 1 cup for the cake batter / ½ cup for the buttercream & ½ cup for the top drizzle
Next preheat the oven to 350F & grease & parchment line your cake pan(s)
Prepare the cake batter by combining the vegetable oil with the melted vegan butter, add the vanilla extract & set aside
In a separate measuring cup combine the plant milk with the vinegar
Sift all the dry ingredients together into a large mixing bowl, including the sugar.
Pour in the wet ingredients (plant milk mixture & the butter/oil mixture including the cooled caramel sauce) into the dry ingredients & whisk smooth about 20 strokes to develop the batter
Pour the batter into your prepared pan(s) & bake for 15 minutes, then turn the open temperature down to 325°F to bake the rest of the way, approximately 20 minutes more or when you gently press the centers they are springy to the touch.
Now prepare the buttercream recipe according to the instructions & add the COOLED ½ cup of reserved caramel sauce at the last stage of mixing
Once the cake is cooled turn it out onto your serving platter & ice with a thick layer of caramel buttercream & then drizzle the top or serve each slice with additional caramel sauce