1. 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.
2. For the croissant French toast, split the croissants in half through the middle.
3. 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.
4. 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.
5. 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.
6. Serve a top and bottom piece together with butter, warm blackberry syrup and maple syrup, strawberries and whipped cream.