Preheat oven to 175°C and line a 12 hole patty pan tray with cupcake papers.
Sift all dry ingredients into a mixing bowl. Then using the stand mixer fitted with the paddle attachment, place the dry ingredients into the mixer and at low speed start adding one butter cube at a time until all incorporated.
Keeping the mixer speed on low, add one egg at a time. Add vanilla extract to milk. Then with mixer speed continuing on low, add milk slowly until all incorporated. Be careful not to flood the batter.
Scrape down the sides of the bowl and paddle then resume mixing on medium-high speed for 30 seconds ensuring the batter is smooth. Then fill cupcake papers ⅔ full and bake for 17-21 minutes until light golden in colour. Transfer to a wire rack and allow to cool completely.
Combine the blackberries and lemon juice in a small saucepan. Stirring constantly, bring to a simmer over medium heat for approximately 6-7 minutes or until the blackberries have softened, and mashed effortlessly into a jam like appearance (equivalent to ¼ cup of blackberry puree).
Strain the blackberry mixture through a fine mesh strainer until it resembles a puree without seeds or any large chunks. Also to cool completely.
Using the stand mixer fitted with the paddle attachment, beat the butter on medium speed until its completely smooth. Then add in the cooled blackberry puree and beat until completely combined. (Don't worry if the mixture looks a little streaky in colour at this point).
Add 1 cup of icing sugar at a time and continue to beat on low speed until fully incorporated. Then add in the salt and increase the stand mixer speed to medium-high to beat for 2 minutes. Then ice the cupcakes as desired and serve.
Cupcakes best stored in an air tight container and consumed within 3 days.