Make the syrup first:
In a medium sauce pan, combine sugar, lemon juice and water and bring to a boil. Reduce heat and simmer for 10 minutes. While simmering, pour berries into a blender and pulse. Pour berry mixture into sugar mixture through a fine mesh strainer to ensure no seeds get through. Use a spoon to stir the berries around so you can press as much juice out of them as possible. Transfer to a jar to finish cooling.
For the doughnuts:
Preheat oven to 350 F
Spray a doughnut pan thoroughly with nonstick cooking spray.
In a large bowl, combine all dry ingredients
In a separate bowl, combine heavy whipping cream, melted butter, vanilla and egg.
Pour 1/2 cup of the reserved blackberry syrup into the wet ingredients.
Fold wet ingredients into dry ingredients
Pour batter into a piping bag and gently fill each doughnut tin with batter. Press a few blackberries into the batter before baking if desired.
Bake for 17 minutes, or until springy
Allow doughnuts to cool completely before icing.
For the glaze:
Combine powdered sugar, lemon juice and zest into a bowl and stir. Spoon over doughnuts and allow glaze to harden just a bit before serving
Enjoy!