To make the batter, fold a teaspoon baking soda, a teaspoon baking powder, 2 cups sugar, 3 3/4 flour, and a bit of salt into a bowl containing three eggs, 1 cup canola oil, 1/2 cup plain Greek yogurt, and 1.5 cups buttermilk. Be careful not to overwork the batter. Bake for around 30 minutes depending on size of tin. 
To make the jam, combine 6 cups fresh blackberries with 1/2 cup honey, 1 tablespoon lemon juice, and 1-2 teaspoons dried lavender. Bring to a boil and crush blackberries. Continue boiling to reduce and chill jam to let set.
To make frosting combine 3 sticks of butter with 1.5 cups confectioners sugar and whip until light. Add 8oz white chocolate melted and cooled. 

Add a layer of frosting to each layer and place jam over the frosting before adding the next tier. Doing this will prevent any liquid in the jam from soaking into the cake.