Combine the flour, brown sugar, cinnamon, nutmeg, and butter in a medium bowl. Mix together with a fork until clumps are formed.
Preheat oven to 350° F. and grease a 9 X 9-inch pan with butter.
In a medium mixing bowl, whisk together the flour, baking powder, and cinnamon.
In a large mixing bowl, cream together the butter and sugar until light and fluffy, approximately 3 to 4 minutes.
Add the eggs and vanilla and mix well, scraping down the sides of the bowl as needed.
Add the flour mixture.
Add the sour cream or Greek yogurt and mix until well combined.
Pour half of the cake batter into the prepared pan and with an offset spatula, spread to cover the bottom.
Sprinkle half of the cinnamon streusel evenly over the cake batter.
Cover with remaining batter and then sprinkle again with the remaining streusel.
Bake for 30 - 35 minutes, until lightly browned and a toothpick inserted into the middle comes out with moist crumbs. Cool completely before slicing.
Mary's recipe in the cookbook called for sour cream, hence the name, Cinnamon Sour Cream Coffee Cake. I substituted plain Greek yogurt as I do in many recipes that call for sour cream because it contains less fat and calories. The taste of the final product was not affected as I have tested it with both, so use whichever you prefer.