For the blackberry syrup: In a saucepan, combine the blackberries, sugar and 1/2 cup water. Bring to a gentle boil and cook for 5 minutes on low. Stir in the cornstarch, then continue cooking, using a whisk or spoon to mash the larger pieces of blackberries. Remove from the heat when it's nice and thick.
For the croissant French toast: Split the croissants in half through the middle.
In a bowl, whisk together the half-and-half, sugar, vanilla, cinnamon and eggs. Dunk each croissant half into the mixture so that it's fully coated. Set the pieces aside on a plate.
Heat a large nonstick skillet over low heat, then melt a small amount of butter in it. Add as many croissant halves as will fit, cut-side down, then increase the heat very slightly (don't go above medium low). Allow the pieces to cook on the first side for 3 to 4 minutes. Move them around in the skillet a bit to make sure they don't burn. When they're deep golden brown on the bottom, flip them to the other side and let them cook for another 2 minutes or so. Remove from the pan and cook the rest of the croissant halves.
Serve a top and bottom piece together with butter, warm blackberry syrup and maple syrup, strawberries and whipped cream.