Preheat oven to 350F. Grease 2, 6" or 7" cake pans w/ butter & dust w/ flour. Line bottoms w/ parchment paper rounds.
Cake Layers: In a bowl, whisk together the oatmilk & apple cider vinegar; rest for 5 mins.
Add pistachios & sugar to a food processor; blend until finely ground.
In a bowl, whisk together the pistachio/sugar mixture, flour, baking powder & salt.
In a separate large bowl, whisk together the oatmilk/ACV mixture, olive oil, & both extracts. Optional: add some natural green food dye to enhance color!
Add dry ingredients to wet; fold to combine.
Divide batter evenly among cake pans (~425g each).
Bake at 350F for 25-30 mins.
Remove & cool COMPLETELY in pans.
Buttercream: In a stand mixer fitted w/ paddle attachment, beat butter w/ blackberry preserves, until smooth.
Gradually add powdered sugar until fully combined. Beat in vanilla, salt & thyme.
Increase speed to medium; beat for 4-5 mins, until light & fluffy.
Assemble: Slice the top of 1 cake layer off to even, leaving the other w/ domed top.
Place evened cake layer on a cake stand. Pipe buttercream in the middle & around edges. Add a spoonful of blackberry preserves to the middle.
Add 2nd layer; pipe buttercream on top. Add a spoonful of blackberry preserves in the middle & garnish w/ blackberries & chopped pistachios.
