Place the frozen raspberries and sugar in a tall saucepan and heat on medium heat until they are softened and reach a boil. Reduce mixture until thick bubbles are bursting on top. (See notes if using fresh raspberries).
Transfer to a blender or food processer and blitz until a smooth puree is formed.
Pass the mixture through a sieve to remove any excess seeds.
Let cool completely before using. The consistency should be kind of similar to ketchup.
Make the Raspberry Cupcakes
Preheat oven to 350°F / 180°C and line a cupcake pan with cupcake liners.
Cream butter and sugar together until pale and fluffy.
Add egg and stir until combined.
Combine flour, baking powder, baking soda and salt in a bowl. Add half of the flour mixture to the batter and mix in completely.
Add the milk, vanilla and ½ cup of the prepared and cooled raspberry puree. Keep the rest of the puree for the filling and frosting later.
Mix in the milk and raspberry completely, then add the remaining flour and stir in. It should be a beautiful pink colour.
Scoop cupcake batter into cupcake liners and bake for 17 minutes or until the tops of the cupcakes spring back when you press them lightly.
Let the cupcakes sit for a few minutes after removing from the oven, then transfer to a wire rack to cool completely.
Once cooled, use a cupcake corer or a spoon to scoop out a hole in the centre of each cupcake and fill with 1 teaspoon of raspberry coulis.
Make sure your butter is at the right temperature, not too hot and not too cold. For best results, remove your butter from the fridge one hour before making the frosting.
Using a stand mixer, whip the butter on high for five minutes, scraping down the sides of the bowl regularly.
Slowly incorporate the condensed milk, whipping in between additions.
Add ½ cup of the prepared raspberry coulis and whip until combined.
Whip for a further minute to allow the frosting to thicken up. It should be beautifully glossy and thick enough to pipe. If it isn't, see notes.
Notes
*All-purpose flour works great for this recipe, but you can also use Cake Flour or Self-raising flour (just leave out the baking powder and baking soda for SR flour). If using fresh raspberries - You can puree the raspberries first if you prefer, and then reduce them, it won't make a difference to the process. If you do cook them first, make sure to add a small splash of water to the saucepan along with the raspberries and sugar. If your Russian buttercream isn't thick and glossy, try one of these things; If it is thin and soupy, the butter was probably too warm - place buttercream in the fridge for 10 minutes then try whipping up again. If it has curdled - the butter was likely too cold - try heating the buttercream very gently over a bain-marie until it just starts to sweat, then whip up for 5 minutes and it should be saved.