Eclair Cake with Chocolate Ganache

Preheat the oven to 400 degrees F (200 degrees C). Grease a 9x13-inch baking dish. Chill a mixing bowl in the freezer.

Combine water, butter, and salt in a medium saucepan over medium-high heat; bring to a boil. Reduce heat to medium; stir in flour. Cook and stir until mixture pulls away from the sides of the pan and forms a ball. Transfer to a mixing bowl and beat in eggs, one at a time, until fully incorporated. Spread dough evenly into the prepared baking dish.

Bake in the preheated oven until golden brown, 25 to 30 minutes. (The dough will rise and make a boat shape, but should drop as it cools.) Cool completely on a wire rack.

Pour 2 cups cold cream into the chilled bowl; whip until cream thickens, about 1 minute. Stir in confectioners' sugar and vanilla. Continue whipping until stiff peaks form. Chill whipped cream in the refrigerator until ready to use.

Combine milk and pudding mix in a separate bowl; stir until creamy. Fold in whipped cream. Spread filling over cooled crust and refrigerate.

Place chocolate in a heat-proof bowl. Bring 1 cup cream in a small saucepan over medium heat almost to a boil; pour over chocolate and let sit for 1 minute, then whisk until smooth. Cool until thickened, about 10 minutes.

Pour chocolate ganache over cream filling, spreading to cover the entire surface. Refrigerate cake before serving at least 1 hour.