In a sauce pan over medium heat whisk together milk, maple syrup, and corn starch. Cook for about one minute to bring up to temp. Stirring frequently, reduce heat to low and cook until thickened, about 3 to 5 minutes. Remove from heat. Stir in the vanilla. Chill uncovered for at least 2 hours. If making in advance, cover once cool.
Make the blueberry sauce:
In a small sauce pan over medium-high heat, whisk together water and sugar until sugar is dissolved. Whisk in vinegar/lemon juice and corn starch. Add the blueberries. Bring to a boil. Reduce heat to a simmer, stirring occasionally, cook for about 10 minutes or until thickened. Set aside to cool. If making a day in advance, refrigerate until ready to use.
Make the French toast:
Preheat oven to 400 degrees. Line two 9 inch cake pans with parchment.
Blend or whisk together the batter ingredients. Transfer batter to a large, shallow dish. Place bread slices in the batter, leaving them just long enough to saturate. Tear or cut bread as needed to fill each pan. If using pre-sliced sandwich bread, versus thicker pieces you sliced yourself, you'll need to make two layers in each pan. Fit torn bread together like a puzzle; this doesn't have to be perfect (see images). Pour any remaining batter over the bread in each pan.
In a small bowl, thoroughly combine all ingredients for the crumble topping. Sprinkle over both pans. Bake French toast 15 - 20 minutes or until the center is hot. Let sit 10 minutes, then remove from pans and let cool to room temperature.
Assemble the trifle:
Cut the French toast cakes into large cubes. In a trifle dish or large glass bowl, add a layer of french toast, then blueberry sauce, then pudding. Repeat - bread, sauce, pudding. Garnish as desired and serve.
Individual components of the trifle can be made a day in advance and assembled just before serving. Once assembled, the trifle is best consumed within a few hours. Refrigerate leftovers for up to 2 days.