Preheat oven to 325°F. Line a 9-inch baking pan with foil and spray with non-stick cooking spray. In a small bowl, combine flour, sugar, cocoa, baking powder, and salt. Cut in the cold cubed butter until the mixture resembles fine crumbs. Mix in egg yolk, vanilla extract, and chopped walnuts, stirring until well combined. Press the mixture onto the bottom of the lined baking pan and bake the crust for 15 minutes.

Beat cream cheese and granulated sugar until smooth in a mixing bowl. Add sour cream, flour, and salt, mixing well. Incorporate eggs and vanilla extract, beating on low speed until combined. Pour the cheesecake mixture over the baked crust and bake for 20-25 minutes. Cool on a wire rack for an hour, then optionally top with chocolate sprinkles. Refrigerate for at least 6 hours or overnight, then lift the foil out of the pan and cut into squares to serve.