Remove the pound cake from its pan and line the pan with plastic wrap, leaving plenty of overhang.
Slice the domed mound off the pound cake and set aside, then slice the remaining cake horizontally into 3 slices.
Melt the butter in a pan over medium heat. Crumble the cake mound into crumbs. Add to the pan and toast until golden brown all over, 1 to 2 minutes. Set aside to cool.
Mix together the blackberries, blackberry jam, lemon zest and juice in a bowl. Set aside.
Combine the heavy cream and powdered sugar in a bowl. Whip until firm peaks form.
Place one layer of cake in the bottom of the lined pan and press to slightly flatten. Top with a third of the blackberry mixture and a third of the whipped cream. Repeat the process 2 more times, ending with whipped cream. Loosely cover with the overhanging plastic wrap and chill in the refrigerator for 8 hours.
To serve, carefully lift the cake from the pan and remove the plastic wrap. Top with the toasted crumbs, a few blackberries and a sprig of mint, then slice and serve.