To make the sauce, combine the water and cornstarch in a small saucepan until dissolved. Add the syrup and berries, and bring to a boil. Continue to cook until berries are mushy and have integrated into the sauce (about 5 minutes).
For the pancakes, mix milk, egg, vanilla, honey, baking powder, and salt in a medium mixing bowl.
Add the cornmeal and whole wheat flour and mix until just combined (it’s OK if the batter is a little bit lumpy).
Heat a cast iron skillet or griddle (or other heavy skillet) on medium high heat and melt 1/2 tablespoon of butter in it.
Add the pancake batter 1/2 cup at a time to the skillet. Flip the pancake when the edges start to solidify and small bubbles begin to appear.
Repeat with the remaining batter, adding more butter for each batch.
Serve pancakes with blackberry syrup.