Preheat your oven to 175°C (350°F) and line a cupcake pan with liners. In a large bowl, whisk together the flour, baking powder, baking soda, salt, and pumpkin spice. In another bowl, combine the dark brown sugar, pumpkin purée, vegetable oil, eggs, and vanilla extract until smooth and well blended. Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Do not overmix. Divide the batter evenly into the cupcake liners, filling each about two-thirds full. Bake for 15-18 minutes, or until a toothpick inserted into the center comes out clean. Remove from oven and let cupcakes cool completely on a wire rack before frosting. For the frosting, beat the butter and dark brown sugar together until creamy. Add the cream cheese and vanilla extract and continue to beat until smooth. Gradually add the sifted powdered sugar and mix until fluffy. Once cupcakes are cool, frost them generously with the brown sugar cream cheese frosting. Serve and enjoy your moist, fluffy pumpkin cupcakes!