How to Make It
Preheat the oven to 375°. Rub the yam with the oil and prick all over with a fork 5 or 6 times. Roast the yam on a rimmed baking sheet for 1 hour, until tender. Let cool slightly, then peel the yam and coarsely mash with a fork.
Reduce the oven temperature to 350°. Spray a 10-by-5-inch loaf pan with nonstick spray and line with parchment paper, leaving 1-inch overhang on the long sides. In a medium bowl, whisk the flour with the baking soda, the 1/2 teaspoon of salt, the ground ginger, cinnamon and black pepper.
In a large bowl, using a hand mixer, beat 1 stick of the butter with the brown sugar at medium-high speed, until light and fluffy, about 3 minutes. Add the eggs, 1 at a time, beating well after each addition. (The batter will look broken.) Scrape 1 1/4 cups of the mashed yam and the grated ginger into the bowl and beat at medium speed until just combined. At low speed, beat in the dry ingredients in 3 additions, mixing until just combined.
Scrape the cake batter into the prepared pan and bake for about 1 hour, until a skewer inserted into the center comes out clean. Transfer to a wire rack and let cool for 30 minutes. Using the parchment paper, lift the yam cake out of the pan and let cool completely on the wire rack.
In a medium bowl, using the hand mixer, beat the cream cheese with the remaining ½ tablespoon of the butter at medium speed until just combined, about 2 minutes. Add the confectioners’ sugar and beat until light and fluffy, 3 minutes. Pour in the milk and beat at medium speed until the liquid is incorporated and the glaze is smooth. At low speed, beat in the lemon juice and a pinch of salt. Pour the glaze over the cake. Transfer the glazed cake to a plate and refrigerate for about 20 minutes, until the glaze is set, before serving.