1. Preheat oven to 200ºF. Set a wire rack over a large baking sheet; place in oven.
2. Combine first 6 ingredients in a bowl; whisk until smooth. Spread evenly over 4 slices of bread. Place remaining 4 slices of bread on top to form sandwiches.
3. In a large dish, whisk milk, eggs and vanilla. Place sandwiches in batter. Let bread soak, about 1 minute per side.
4. Melt 2 Tbsp. butter in a nonstick skillet over medium heat. Add 2 sandwiches; cook, flipping halfway through, until golden brown on both sides, 6 to 8 minutes total. Transfer to rack on sheet in oven. Repeat with remaining sandwiches.
5. Top French toast with pecans and maple syrup, if desired, and serve.