Melt ½ cup butter (in a small saucepan or microwave). Stir in 1 cup brown sugar until combined. Pour this mixture into the bottom of a 9×13-inch casserole dish and spread evenly.

Trim crusts from the Texas toast if desired and cut slices as needed to fit the pan. Arrange a single layer of Texas toast over the butter-brown sugar mixture.

In a small bowl, combine 4 tablespoons brown sugar and 3 teaspoons cinnamon; set aside.

In a medium bowl, whisk together 4 eggs, 1½ cups milk, and 1 teaspoon vanilla until blended.

Pour half of the egg mixture evenly over the first layer of Texas toast. Press the bread down gently so it soaks up the custard. Sprinkle half of the cinnamon–brown sugar mixture evenly over the soaked bread.

Add a second layer of Texas toast, trimming to fit if needed. Spread 4 tablespoons softened butter thinly and evenly over the top layer.

Pour the remaining egg mixture evenly over the top layer. Sprinkle the remaining cinnamon–brown sugar mixture evenly over the top.

Cover the dish and refrigerate for at least 4 hours or overnight.

Preheat the oven to 350°F. Remove the cover and bake uncovered for 40–45 minutes, until the custard is set and the top is golden.

Let cool slightly, then sprinkle with powdered sugar before serving.