Melt the Butter: In a medium saucepan over low heat, melt the butter until fully liquefied. Add milk, water, and salt, then bring to a gentle boil while stirring.

Mix Dry Ingredients: In a large mixing bowl, combine flour and baking powder. Whisk together until evenly blended.

Combine Ingredients: Add the dry flour mixture to the saucepan once the liquid has boiled and stir continuously until a dough forms.

Cool Dough and Add Eggs: Transfer the dough into a large bowl to cool for about 10 minutes. Add eggs one at a time, mixing vigorously after each.

Bake Layers: Preheat oven to 400°F (200°C). Divide the dough into two portions, spread in a baking pan, and bake for 20 minutes.

Prepare Cream Filling: Beat eggs and sugar together, then whisk in milk and flour. Cook over medium heat until thickened.

Make Cream Mixture: Cool the cream filling. In a separate bowl, beat softened butter until fluffy, then incorporate the cooled cream.

Assemble Cake: Slice baked layers and layer with cream filling between each layer.

Chill and Serve: Cover with plastic wrap and refrigerate for at least 2 hours. Dust with icing sugar before serving.