This unbelievably tender pumpkin cake is frosted with salted maple buttercream and sprinkled with maple-candied pepitas. The batter comes together in one bowl, so you have more time to make your homemade frosting and candied topping. While making your frosting, look for pure maple syrup with a dark color and robust flavor. You can add a ½ teaspoon of maple extract, but we find the artificial flavor to be overpowering.
A note on aluminum versus glass bakeware: We like to use a metal pan for things like cakes and brownies because they conduct heat faster, giving you a more consistent rise and color on your baked goods. Glass baking dishes, by comparison, are heavier than aluminum and more expensive. Because glass is an insulator, rather than a conductor, it’s slow to heat and, once hot, retains that heat longer. So, by the time the interior of your cake is baked through, the exterior is often overcooked, dry or dark.
Preheat the oven to 350°F. Grease a 9-by-13-inch metal baking pan and line with parchment paper, leaving a slight overhang on two sides.
In a large bowl, whisk the pumpkin purée with the oil, eggs, both sugars and vanilla extract until smooth. Add the flour, baking powder, baking soda, salt and spices and gently mix until there are no lumps. The batter should be thick. Scrape into the prepared pan and smooth the top. Bake for 30 to 35 minutes, or until a toothpick inserted in the center comes out clean. Transfer the cake pan to a wire rack and let cool completely.
Make the pepitas
Line a plate with parchment paper. In a small skillet, toast the pumpkin seeds over moderate heat, stirring occasionally, until fragrant and beginning to brown, 3 to 5 minutes. Transfer to a small bowl. In the same skillet, add the maple syrup—it should begin to bubble rapidly. Cook for 30 seconds over moderate heat without stirring. Remove the skillet from the heat and stir in the toasted pepitas with a heatproof spatula until they’re evenly coated. Transfer to the prepared plate and sprinkle with the flaky sea salt. Let cool completely, then break into small clumps.
Make the frosting
In a large mixing bowl and using a hand mixer, beat the butter on moderately-high speed until fluffy. Add the maple syrup, vanilla and salt and beat until smooth. Add the confectioners’ sugar and 2 tablespoons of the cream and, beginning on low and gradually increasing the speed to moderately-high, beat until smooth. Scrape down the sides as needed and be careful not to over-beat your frosting. If your frosting is too thin, gradually add more confectioners’ sugar. If it is too thick, add the remaining heavy cream.
Using the parchment paper, lift the cooled cake from the pan and transfer to a platter. Spread the buttercream on top in an even layer. If desired, transfer the cake to the refrigerator for 30 minutes to firm up the frosting. This will make the cake easier to slice. Garnish with the candied pepitas, slice and serve.
MAKE AHEAD The frosted cake can be stored in the refrigerator for up to 4 hours. Let sit at room temperature for 20 minutes before serving to let the frosting soften slightly. Garnish with the candied pepitas before slicing and serving. The pepitas can be made up to two days in advance and stored at room temperature in an airtight container.