Preheat oven to 325 degrees. Line a baking sheet with parchment paper and set aside.
In a large food processor fitted with a dough blade, pulse together the cake flour, baking powder and caster sugar.
Add the butter and mix until it resembles chunky bread crumbs.
Add the buttermilk, egg, vanilla bean paste, lemon zest and lemon juice and pulse the dough until just combined.
Turn the dough onto a floured surface and, using your hands, gently incorporate the the frozen raspberries into the dough.
Dust the dough with flour and flatten it into a 1-inch-thick round. Using a sharp knife, cut into wedges.
Place the wedges on the prepared baking sheet, brush with soft butter and sprinkle with raw sugar.
Bake for 22 - 24 minutes or until light golden brown.