For the cookie dough: 

1. Cream softened butter with light brown sugar, agave, and vanilla until light and fluffy. There shouldn't be any lumps. 
2. In a separate bowl, combine the dry ingredients. 
3. In parts, add dry ingredients to the wet. 
4. Once mixed, fold in chocolate. Add nuts if desired.

Assembly: 

1. Line the bottom of an 8x8 pan with parchment paper. Using your hands, spread 1/2 of the dough mixture evenly across the pan. If you're struggling to keep the parchment paper in place, use double sided tape, or pieces of doubled over tape to hold down the corners. 
2. Put another piece of parchment paper on top of the first layer and spread on the next layer. Then freeze for at least 3 hours. I froze mine for 24. 
3. When you're ready to add the ice cream, soften at room temp for 5-10 minutes. When the ice cream is ready, remove the dough from the freezer. Run a table knife along the edges of the pan to loosen the dough sheets. Carefully remove the top layer, and discard the parchment paper from the middle. Spread the ice cream evenly, replace the top layer, and gently press down. Cover, and let freeze for another 3 hours minimum. 
4. When ready to eat, do the knife along the edge thing again before turning the pan over on a cutting board. You may need to tap the bottom of the pan a little. Cut and enjoy!