In a big bowl, mix together flour, sugar, and salt. Pour in lukewarm milk, melted butter, and beaten egg. Mix until well blended. Add the instant yeast and mix well to form a slighly sticky dough.
Tip the dough on a surface greased with oil. Knead the dough with lightly oiled hands until it becomes smooth and elastic, about 10-12 minutes.
Form the dough into a ball and place the dough in a greased bowl. Cover with a kitchen towel or plastic wrap and place it in a warm area and let it rise until it doubled in size. Depending on how warm it is, could take 30 minutes to an hour or even longer.
Meanwhile, prepare the filling. In a saucepan over medium-low heat, melt butter until bubbly. Add the ¼ cup bread crumbs and mix well. Add flour, water or milk, sugar, salt and rest of breadcrumbs. Cook until it gets thick. Remove from heat and continue stirring until it forms a thick paste-like texture.
Punch down the dough and divide into 16 equal parts using a knife of dough slicer. Roll each piece into a long triangle (like a pizza slice). Brush the middle part with butter or margarine then spread filling on the surface leaving about a centimeter around the edges unfilled. Roll the dough starting from the wider end towards the smaller end enclosing the filling.
Brush each rolled dough with milk or water then roll it on a plate of breadcrumbs and place on baking sheet lined with parchment paper: make sure that the end part sits at the bottom so it won't open up while baking. Cover with cloth or kitchen towel or cling wrap and let them rise for another 20-30 minutes or until they doubled in size. Remove cover before baking.
Place the baking sheet on the middle rack and bake at 150°C (300°F) for 20 minutes.
Remove the Spanish bread from the oven and transfer to a bread basket. Enjoy while still warm.