Combine blackberries, lemon juice and zest, sugar, vanilla extract, and water in a small saucepan. Bring to a boil over medium-high heat.
Add cornstarch, stir, and simmer until the compote thickens. Remove from heat.
In a large bowl, whisk together eggs, milk, cinnamon, and vanilla extract.
Spread whipped cream cheese on one side of each bread slice. Optionally, spread a tablespoon of compote over cream cheese.
Melt 2 tablespoons of butter in a skillet over medium-high heat.
Dip sandwiches in egg mixture, place in skillet, and cook until golden brown on both sides, about 2 minutes per side.
Repeat with remaining sandwiches, refreshing butter as needed. Serve warm, topped with additional compote and a dusting of powdered sugar.
