Pour a few tablespoons of the milk into a measuring cup. Add the all-purpose flour and whisk thoroughly until the mixture is smooth and no visible lumps remain.
Stir this slurry into the remaining milk, blending gently until evenly combined. This preparation helps the custard cling consistently to the delicate cake slices.
Crack the eggs into a large mixing bowl. Add the prepared milk-and-flour mixture, vanilla extract, salt, and ground cinnamon.
Whisk until the ingredients are fully incorporated and the custard has a uniform consistency.
Lift the slice from the bowl and allow the excess custard to drain away. Avoid leaving the cake submerged for too long, as its airy texture may soften excessively and break apart.
Warm a large skillet or griddle over medium to medium-low heat. Melt a small portion of the butter across the cooking surface.
Arrange the custard-coated cake slices in the heated skillet without overcrowding. Cook each broad side for 2 to 3 minutes, or until a crisp, golden exterior develops.
Carefully stand each slice on its thicker third edge and toast that surface until lightly browned.
Continue cooking the remaining pieces in batches, adding more butter whenever necessary.
Transfer the warm angel food cake French toast to serving plates.
Finish each portion with sliced strawberries, fresh blueberries, whipped cream, and optional mint leaves. Drizzle with pure maple syrup immediately before serving.