Preheat oven to 350ºF.
Line a baking sheet with parchment paper.
In a bowl combine the flour, baking soda and salt.
In another bowl, cream the butter with white sugar until light and fluffy.
Add the egg, lemon juice, vanilla extract, lemon zest and poppy seeds and beat to combine.
Add the flour mixture and mix to combine.
Drop the dough by rounded tablespoons onto the prepared baking sheet.
Bake in preheated oven for 11-12 minutes or until lightly golden.
Remove from the oven and allow to cool completely.
To make the frosting, in a small bowl cream the cream cheese and butter.
Add the mashed blackberries, vanilla extract and cornstarch.
Add the powdered sugar and beat to combine.
Pipe the frosting onto cooled cookies.
Top each cookie with 1 teaspoon of blackberry jam.
Refrigerate until ready to serve.