Mix warm milk with yeast and sugar.
Let mixture sit for 5 minutes until foamy.
Add flour, egg, butter, and salt to bowl.
Mix until smooth dough forms completely.
Cover dough and let rise for 1 hour.
Press dough into greased springform pan evenly.
Combine topping ingredients in small saucepan carefully.
Cook topping until golden and bubbly appears.
Spread hot topping over dough immediately.
Bake at 350°F for 25 minutes total.
Cool completely before removing from pan carefully.
Slice cake horizontally into two equal layers.
Prepare custard filling by whisking all ingredients.
Cook custard until thick and smooth consistency.
Spread custard between cake layers evenly.
Refrigerate assembled cake for 2 hours minimum.