Preheat oven to 350 degrees.
Grease your baking pans, two round tins 6-8 inches. Smaller pans will produce thicker cakes. I grease with a bit of oil and a pinch of flour Parchment sheets can help line the bottoms of pans.
Add all the wet ingredients to a large mixing bowl. Mix a bit.
Add in the dry ingredients. Mix until smooth and fluffy.
Pour batter into baking tins.
Bake at 350 for 22-30 minutes, depending on pan size. My 7 inch pans did about 27 minutes.
Remove cakes and allow them to completely cool. A few hours or overnight. If making the day before, wrap slightly cooled cakes in plastic wrap and place in fridge until morning.
Frosting can be made ahead of time and re-whipped, or made same day. The main rule is to only whip when the frosting or shortening is about room temperature. That ensures a fluffy easy whip. Add shortening, vanilla and oat milk to mixing bowl. Whip with a whipping attachment. Then add in the sugar and whip from low to high until fluffy. A few minutes should be good.
Frost your cake in layers. Use the flat side or the cake on the top and bottom for an even frost on top. Keep cake at room temperature for a few hours, if serving right away. Or place in fridge to chill. Just allow the frosting time to soften before serving.