1.Preheat oven to 180°C/350°F. Grease and line a 22cm (9in) loose-based tart tin.
2.Pulse oats in a food processor until they resemble flour. Transfer to a large bowl; sift in almond meal, cocoa and baking powder.
3.Place butter, maple syrup, sour cream, egg and egg white in a medium bowl; whisk until combined. Fold egg mixture into oat mixture until combined; transfer mixture to tin.
4.Spread oat mixture over base of tin, creating a 2cm (¾in) rim up side of tin to form a ‘crust’ around the edge. Bake for 15 minutes or until firm and dry; cool completely in tin.
5.Meanwhile, mix yoghurt and vanilla bean paste in a small bowl until combined; refrigerate until required.
6.Melt chocolate. Remove cookie base from tin; spread with yoghurt mixture. Arrange cherries and berries on top. Drizzle with chocolate and sprinkle with mint to serve.