Sprinkle the yeast over the warm water in a small bowl and let it sit for 5 minutes until it foams. If it doesn't foam, your yeast is dead — start over with fresh yeast.
Pour the milk into a small saucepan and heat just until bubbles form around the edges — don't let it boil. Remove from heat, add the shortening, and stir until melted. Let this mixture cool to lukewarm before proceeding.
Whisk the eggs in a large mixing bowl until smooth. Add the salt, sugar, cooled milk mixture, and activated yeast. Stir everything together until well combined — the mixture should be uniform with no streaks.
Add the flour one cup at a time, mixing with an electric mixer on low speed after each addition. The dough will be quite soft and slightly sticky — this is exactly what you want for tender coffee cake.
Scrape down the sides of the bowl thoroughly, cover with a damp kitchen towel, and place in a warm spot to rise for 2 hours or overnight until doubled in size. The long rise develops the flavor beautifully.
⏱ 2 hrs
Turn the risen dough onto a well-floured surface and knead it exactly 200 times — count them out. This develops the gluten structure and makes the dough easier to handle.
Divide the dough in half using a bench scraper or knife. Roll each half into a rectangle that fits your prepared pans — the dough should be about ½-inch thick.
Transfer each piece of dough to a greased 9x13-inch pan, gently stretching to fit if needed. The surface should be relatively even for the topping to spread properly.
Beat the softened butter with an electric mixer until light and airy — about 3 minutes. Gradually add the sugar and continue beating until the mixture is pale and fluffy.
Add the eggs to the butter mixture one at a time, beating well after each addition. Mix in the powdered milk, flour, and vanilla, beating until smooth. The topping should be thick but spreadable.
Divide the topping between the two pans and spread it evenly over the dough using an offset spatula. Cover and let rise in a warm place for 2 hours until the dough looks puffy and light.
⏱ 2 hrs
Bake in a 350°F oven on the middle rack for 12-15 minutes, until the topping is set and just beginning to turn golden brown. Don't overbake — the center should still have a slight jiggle when you gently shake the pan.
⏱ 15 min