Place the blackberries and sugar in a small saucepan. Cook over medium heat until the berries begin to break down, then lower the heat and simmer. Crush the berries with the back of a wooden spoon and cook until slightly thickened. Strain over a small bowl, pressing the fruit with the back of a spoon to extract all the juice. Cool in the fridge until ready to use.
Wipe a stainless mixing bowl and a whisk attachment clean with a paper towel and a little lemon juice or vinegar. Place the egg whites in the mixing bowl but do not turn the mixer on.
In a medium saucepan over low heat, stir the sugar and water together, then stop stirring and brush any sugar crystals off the side of the pan and down into the syrup with a damp pastry brush. Clip on a candy thermometer and heat to 240 degrees.
While the syrup is cooking, whip the egg whites on low speed until foamy, then add the cream of tartar, increase the speed to medium and gradually add the remaining tablespoon of sugar. Beat until soft peaks form. Continue beating until the syrup is ready.
Once the syrup reaches 240 degrees, immediately remove the pan from heat, turn the mixer on high speed and slowly and steadily pour the hot syrup down the side of the mixing bowl between the whisk and the bowl to prevent the hot syrup from splashing out. Beat until the meringue is stiff and glossy and the bowl has cooled to room temperature.
Switch to the paddle attachment and turn the mixer on low speed. Slowly add the cubes of butter. Increase speed to medium and mix until the buttercream is smooth and fluffy.
Pour in the cooled blackberry syrup and mix to combine. Add the ground vanilla beans, extracts and a pinch of salt and mix until the buttercream is smooth.
Recipe by oh honey bakes at https://ohhoneybakes.com/lemon-poppyseed-cake-with-blackberry-vanilla-bean-buttercream/