Not creaming the fat long enough (creaming
cuts little air pockets into the fat, which means the leavening has to do less work)
You want to avoid over-proofing the dough (letting it become too puffy so that it collapses during cooking), but allowing the dough sufficient time to build up lots of
little air pockets will help give you the crumb texture you're looking for.
My muffins developed
a little air pocket in the center, which was from some of the dulce de leche getting absorbed into the muffin after it was done baking aka making it super moist and decadent.
Before you seal the top, be sure to leave
a little air pocket so the filling has room to steam.
If the egg stays a quarter way above the base of the glass or floats to the middle,
a little air pocket has formed, inside in the skin, and these eggs are best used for hard or soft cooked eggs, producing an even better result than entirely fresh eggs.