Combine all the streusel ingredients in a medium bowl. Work the mixture with a fork or pastry cutter until mostly mixed. Finish with hands, working until the butter is all mixed in and large clumps form when you squeeze it together. Cover and refrigerate.
For coffee cake: Preheat the oven to 375 F/190 C, and butter and flour a 13 x 9 baking pan. For muffins: preheat oven to 400 F/204 C, and line two 12-cup muffin pans with baking cups.
Beat softened butter with maple syrup and sugar, then add eggs and vanilla. Beat well.
Add yogurt, flour, baking powder, baking soda, and salt. Beat on medium until light and fluffy.
For coffee cake: spread batter into prepared pan, then sprinkle streusel on top, breaking up any large clumps. Bake for 40-45 minutes, until a toothpick inserted in the center comes out clean. Cool completely, or enjoy while still warm.
For muffins: divide batter evenly amongst the 24 cups. Break up any large clumps of streusel, then sprinkle evenly over the tops of the muffins. Bake for 16-18 minutes on separate racks in the oven, rotating once, until a toothpick inserted in the center comes out clean. Cool on a wire rack.
Notes
To make this gluten free, simply substitute the flour with a one-to-one GF baking flour and GF oats. If you don't have certified GF oats, substitute with extra flour and/or chopped nuts. (Thanks to Katrina for the GF tips!)Adapted from Maple Muffins with Oat Streusel