Preheat the oven to 400 degrees . In a large bowl, combine the flour, baking powder and cinnamon. Place the peanut butter in a medium bowl and microwave at high power to soften, 5 to 10 seconds. Whisk in the milk, then stir the mixture into the dry ingredients.
Shape the dough into a long, flattened log and place on a greased baking sheet. Bake for 20 minutes; remove from the oven and let cool for 10 minutes. Slice the log crosswise on an angle into 1-inch-thick pieces. Set the slices on their sides (they should look like biscotti) and bake until crisp, 10 to 12 minutes more. Transfer to a rack to cool.
Melt the carob in a microwave or on the stovetop over low heat. Thin the melted carob with 1 tablespoon water, if needed. Using a spoon, drizzle the melted carob over the biscuits. Let cool completely. Store in an airtight container.