Reduce the apple cider. Start by pouring 3 cups of apple cider into a medium saucepan. Place the saucepan over medium heat. Bring it to a simmer and let it simmer until it has reduced down to 1/2 cup, stirring occasionally. This reduction can take about 15-20 minutes. The goal is to concentrate the apple flavor, so the cider should look thicker and more syrupy when finished. Once reduced, remove from heat and let it cool to room temperature.
Cream the butter and sugars. In a large mixing bowl, combine softened unsalted butter, granulated white sugar, and light brown sugar. Use an electric mixer or whisk to beat the mixture until it becomes light and fluffy. This process helps the donuts turn soft and airy.
Mix in the apple cider, eggs, and remaining liquids. Add the cooled, reduced apple cider, egg, and vanilla extract to the butter-sugar mixture. Mix until well combined. Then, slowly pour in buttermilk, stirring gently to prevent curdling.
Combine the dry ingredients. In a separate bowl, whisk together all-purpose flour, baking powder, baking soda, a pinch of salt, ground nutmeg, ground cinnamon, and ground allspice.
Combine the dry ingredients with the wet ingredients. Gradually add the dry ingredients to the wet mixture, stirring gently until just combined. Avoid overmixing as this can make the donuts dense. The batter should be thick and slightly sticky.
Prep the oven and donut pan. Preheat the oven to 350°F. Grease the wells of a donut pan with nonstick spray or a little melted butter to ensure the donuts release easily after baking.
Add the doughnut batter to the pan. Transfer the batter into a piping bag or a zip-top bag with a corner snipped off. Pipe the batter evenly into the donut wells, filling each one about ¾ full.
Bake and cool. Bake the donuts for 10-12 minutes or until they spring back when gently pressed. Be careful not to overbake as this can make them dry. Once done, remove them from the oven and let the donuts cool in the pan for 5 minutes, then transfer them to a wire rack to cool completely.
For the Spiced Sugar Topping
Make the cinnamon sugar. In a shallow bowl, mix granulated sugar, ground cinnamon, and ground allspice. Melt butter in a separate bowl.
Coat the donuts with the cinnamon sugar. Once the donuts have cooled slightly, brush each donut with melted butter, then dip them into the spiced sugar mixture. Make sure to coat both sides generously for the best flavor.
Serve. Serve the donuts warm or at room temperature. These soft apple cider donuts pair perfectly with a cup of tea, coffee, or hot cider. Enjoy!