Warm the water and sugar in a small saucepan over medium heat. While stirring, bring the mixture to a boil, then remove the pan from the heat. This is simple syrup. Measure out 1/4 cup of the syrup and whisk it into the oil and peanut butter in a small bowl. Allow to cool.
Preheat the oven (a toaster oven works great for this) to 400 degrees. Butter 1 side of each bread slice, sprinkle with a bit of salt, and place the bread on your oven rack. Bake until the slices are dark golden brown, and the edges are slightly darker. The bread should be crisp throughout, no soft spots. Let the slices cool, then use a knife or your fingers to crumble them into roughly pea-size pieces.
Blend the milk and ice cream until it is just incorporated. Fill each glass halfway with the shake mixture. Then scoop some strawberry syrup and peanut butter swirl into each glass. Pour the remaining shake mixture over the peanut butter swirl and strawberry syrup.
Garnish with a drizzle of both the strawberry syrup and the peanut butter swirl, and a generous portion of toasted bread crumbles. This shake is best eaten with a spoon!