Make Blackberry Sauce. Cook blackberries, sugar, and water over medium-low heat in a small saucepan for about 10 minutes. Mash the blackberries up as they cook. Let cool.
Process mixture in a blender until smooth. Strain through a fine-meshed sieve. Discard the solids.
Cut 1/2 an inch off the top of the cake. Hollow out the cake leaving 1/2-inch border on the sides and bottom.
In a large bowl, beat cream, confectioners' sugar, and almond extract with an electric mixer until stiff peaks form. Fold in sour cream.
Drizzle 1/4 cup of Blackberry Sauce in the hollowed out part of the cake. Scatter the blackberries in the cake. Add whipped cream mixture to fill the hollowed out portion.
Swirl the remaining Blackberry sauce into the remaining whipped cream mixture.
Place the top of the cake back on and spread whipped cream mixture on the top and sides of the cake.
Refrigerate for at least one hour before serving.
Garnish with blackberries and mint leaves.
