2 boxed brownie mixes, prepared in a 9×9 pan according to package directions
For the ice cream center
Line a 9×9 pan with plastic wrap, making sure it goes all the way up all sides! Meanwhile, allow ice cream to sit out at room temperature while you make your blackberry purée.
In a small saucepan over low heat, cook half of the blackberries with lemon juice until blackberries release their juices, about 20 minutes. To help this process, smoosh blackberries with a fork to break them up.
Allow mixture to cool to room temperature, then strain blackberries through a fine mesh strainer, saving the liquids and discarding the solids. You should end up with about 1/4 cup of liquids.
Remove ice cream from container and place into a large bowl. Mix in blackberry liquids with a whisk until well combined. Pour into plastic-wrap-lined pan, then place remaining whole blackberries into mixture and freeze overnight.
For the rest
Prepare boxed brownie mixes according to package directions. Make sure to use a 9×9 pan, as this is the same size pan you’re using for your ice cream.
Allow brownies to cool completely, then, very carefully, cut in half.
Place one layer of brownie on a large cutting board. Top with frozen blackberry ice cream, then finish with top layer of brownie! Place everything into the freezer for 30 minutes more, then cut into squares and serve!!!