Preheat the oven to 350°F (175°C) and grease a 9-inch round cake pan.
In a bowl, whisk together flour, baking powder, baking soda, salt, and espresso powder.
In another bowl, beat softened butter and brown sugar until fluffy. Add eggs and vanilla, mix well.
Slowly add dry ingredients and sour cream, mixing until just combined.
Fold in chopped pecans and pour the batter into the prepared pan.
In a separate bowl, combine streusel ingredients: oats, brown sugar, flour, butter, and cinnamon.
Sprinkle streusel over the batter.
Bake for 35-40 minutes or until a toothpick comes out clean.
Let cool on a rack before slicing and serving.