Prepare the biscuits. Slice each of the 8 day-old biscuits horizontally through the center to create a top and bottom half. Place all 16 pieces aside until ready to soak.

Prepare the custard. Add the eggs, whole milk, vanilla extract, and ground cinnamon to a mixing bowl. Whisk thoroughly until the mixture is smooth and evenly blended.

Heat the skillet. Set a skillet over medium-high heat and add a generous amount of butter. Allow it to melt and spread across the entire cooking surface. The butter should be sizzling before cooking begins, but it should not be allowed to brown.

Soak the biscuit halves. Dip the biscuit pieces into the prepared egg mixture one at a time. Leave each piece in the custard for several seconds so it can absorb some of the liquid without becoming overly saturated. Lift it out and gently shake away any excess mixture.

Cook until golden. Arrange the soaked biscuit halves in the heated, buttered skillet without overcrowding the pan. Cook the first side for approximately 2–3 minutes, or until golden brown and set. Turn each piece over and cook the second side until similarly browned and cooked through. Work in batches as necessary, adding additional butter to the skillet between batches when needed.

Serve immediately. Transfer the finished cinnamon French toast biscuits to serving plates while they are still hot. Add more butter if desired and serve with preferred toppings, such as syrup and fresh berries.