In a medium saucepan over low-medium heat, add the frozen raspberries, lemon juice and 1 tablespoon of sugar. Bring to a simmer and continue to simmer while stirring and crushing the raspberries for about 15-20 minutes until thick.

Strain the puree making sure to press as much through the sieve as possible. Cool to room temperature before using.

Preheat your oven to 350F / 180C / 160C fan and line muffin tins with paper cases.

Sift together flour, cocoa, baking powder, baking soda, and salt.

In a separate bowl, beat together the melted butter and sugars until combined.

Add the eggs one at a time, mixing well after each.

Mix in vanilla.

Alternately add the flour mixture and buttermilk until just combined.

Fill cupcake cases to about ⅔ full and bake for 18-23 minutes.

Cool in the tin for 5 minutes before transferring to a wire rack.

Whisk the flour and some milk together in a small saucepan to make a smooth paste, then add remaining milk and ½ cup sugar. Heat over low heat until thick.

Cool this mixture to room temperature.

Beat together the butter and remaining sugar until creamy, about 5 minutes.

Add cooled milk mixture gradually, mixing well after each addition.

Once combined, mix on medium-high speed until fluffy.

Add vanilla, raspberry puree, and salt. Mix on low just to incorporate.

Cut a 1 inch hole in the top of each cupcake and spoon in 2 teaspoons of raspberry puree.

Pipe or spread frosting on top and top with a fresh raspberry.