Heat oven to 350 degrees. Spray a 9x13-inch baking pan.
In a medium bowl, whisk the flour, baking powder, baking soda, salt and spices until well combined.
In the bowl of a stand mixer fitted with the paddle attachment, combine the brown sugar, butter and grapeseed oil. Beat on medium-high until light and fluffy, about 3 minutes. Add the eggs one at a time, mixing for about 20 seconds between each. Add the vanilla extract and pumpkin purée and mix until well combined, scraping the bottom and sides of the bowl as necessary.
Turn the mixer to low, and add the dry ingredients. Stir until a few spots of flour remain. Remove the bowl from the mixer, and use a rubber spatula to finish folding in the dry ingredients. The batter will be thick and fluffy, so make sure to scrape the bottom and sides of the bowl to ensure it is evenly mixed.
Pour the batter into the prepared pan, smooth the top, and firmly tap the pan on the countertop a few times to release any large air bubbles. Bake the cake until golden and puffed, and a tester inserted into the center comes out clean, 25 to 30 minutes.
Cool the cake, still in the pan, on a rack for 15 minutes, then use the tip of a knife to loosen the edges and carefully invert it onto the rack to cool completely.
Make the frosting: Add the cream cheese and butter to the bowl of a stand mixer fitted with the paddle attachment. Mix on medium-high until smooth. Turn the speed to low and slowly add the confectioner’s sugar, mix until the sugar is moistened, then turn the mixer up to medium-high and whip until smooth. Add the molasses, lemon juice and vanilla, and whip until smooth.
Invert the cooled cake to a serving platter, frost and serve. Alternately, you can also frost and serve this cake in the pan it was baked in, if you’d rather not transfer it to a serving platter.