Preheat oven to 350F. Line a baking sheet with parchment paper. In a small bowl, combine all streusel ingredients until crumbly. Spread in a single layer on prepared baking sheet. Bake at 350F 8-10 minutes until a light golden brown. Set aside to cool.
Pancake Batter
In a large skillet over medium-high heat, heat 1 tablespoon butter until pan is hot then add apples. Saute apples, stirring occasionally until apples are softened and tender. Remove from heat and cool.
In a large bowl, whisk flour, baking powder, baking soda, cinnamon, salt, and sugar. In a separate bowl, whisk egg, remaining 3 tablespoons melted butter, and buttermilk until smooth. Fold flour mixture into liquid until batter is just combined. Fold in 1 cup sauteed apple mixture (reserve the rest of the apples for topping the pancakes)
Working in batches, drop pancake batter by 1/3 cupfuls onto a greased hot skillet. Sprinkle streusel over first uncooked side of each pancake and let pancakes cook 2-3 minutes per side, flipping once. Keep pancakes warm.
Spiced Maple Syrup
Combine all syrup ingredients in a small saucepan and heat over medium-high heat until just simmering.
Serve pancakes warm with reserved apple mixture, any remaining streusel, and warm spiced maple syrup on top. Enjoy!