Preheat the oven to 350°F with a rack in the middle position. Lightly grease one 8 1/2" x 4 1/2" loaf pan or a loaf pan with at least a 6-cup capacity. Place the pan on a baking sheet for additional stability (especially if using a specialty loaf pan).  , To make the cake batter: In a large bowl or in the bowl of a stand mixer fitted with the flat beater, beat the eggs and sugar until smooth. Gradually add the vegetable oil, beating all the while. Stir in the vanilla, baking powder, and salt.  , Gently mix in the flour, scraping the bowl as needed to fully incorporate, until a stiff batter forms and no dry ingredients remain. , Transfer half of the batter (about 1 cup or 350g) into a medium bowl. Mix in the pumpkin purée and pumpkin pie spice just until combined. Set aside. , To the remaining batter, add the sour cream and beat just until incorporated, less than 1 minute., Alternating between the two batters, dollop roughly 2-tablespoon portions of batter into the prepared loaf pan to create a checkerboard effect. (Precise measurements are not important here; the beauty of this cake comes from a more free-form approach.)  , Once all the batter has been added to the pan, insert an offset spatula or wooden skewer deep into the batter and drag it through in one decisive figure-8 motion. Less is more; one figure 8 will provide a nice swirl in the cake when it’s sliced without mixing the batters too much.  , Bake the pumpkin swirl cake for 45 to 55 minutes, or until a paring knife inserted into the center comes out clean or with a few moist crumbs attached. , Allow the cake to cool in its pan for about 5 minutes before turning it out onto a wire rack to cool completely. While the cake cools, make either the soak or the glaze. (See “tips,” below for how to choose.) , To make the soak or glaze: In a small bowl, whisk together the hot water and sugar, stirring to dissolve the sugar. Add the vanilla. Brush the soak onto the cake. Alternately, prepare the glaze instead: Whisk together the confectioners’ sugar, water, and vanilla to form a thin glaze. Slowly pour the glaze over the cooled cake to coat the top surface and allow some small drips to fall over the sides. , Storage instructions: Store leftover pumpkin swirl cake in an airtight container at room temperature for up to 4 days. Freeze for longer storage.