Preheat oven to 180ºC. Grease a square brownie tin and line with baking paper.
In a bowl, add butter and chocolate and microwave in 60 second increments for 2-3 minutes. Stir with a metal spoon in between until butter and chocolate become smooth and silky. Allow to cool for five minutes.
Whisk in egg, Natvia, flour, cocoa powder, vanilla and salt until just combined. Add frozen raspberries and stir quickly to evenly distribute among the batter.
Pour into prepared tin and bake for 40 minutes. Allow to cool completely before slicing and serving.