Preheat oven to 350 degrees F.
Prepare Blackberry Topping (see below); set aside.
In a large bowl, combine flour, sugar, baking powder, and salt. Add egg, milk, and butter; beat 2 minutes. Add lemon zest, lemon juice, and vanilla extract; beat 2 minutes. Pour over blackberries in frying pan, spreading evenly.
Bake 40 to 50 minutes or until a toothpick inserted in center comes out clean.
Cake is done when the internal temperature registers approximately 190 to 200 degrees F. on your cooking thermometer.
Remove from oven and cool 5 minutes on a wire rack.
Run knife around edge of pan to loosen; cover with a cake plate and invert. Serve warm.
Makes 8 servings.
In a 10-inch cast iron frying pan or a 9x2-inch round cake pan over low heat, melt butter. Stir in brown sugar until blended. Remove from heat.
Sprinkle lemon zest over the brown sugar. Sprinkle chopped nuts over the top. Arrange the blackberries in an even layer over the nuts; set aside.
Blackberry Upside-Down Cake Recipe: https://whatscookingamerica.net/fruit/blackberryupsidedowncake.htm