Position a rack in the middle of the oven and preheat to 350°F. Oil the sides and bottom of the cake pan and line the bottom with a parchment circle or paper cut to fit. Do not oil the paper.
Pour the Guinness or other dark stout into a medium saucepan and heat over medium heat. Simmer for about 15 minutes or until Guinness turns syrupy and measures 3/4 cup.
Place a wire mesh strainer over a medium bowl. Add the pastry flour, all-purpose flour, cocoa, sugar, baking powder, baking soda, and salt to the strainer and stir with a whisk to sift the ingredients into the bowl. (If any small bits remain in the strainer, add them to the mixture in the bowl.) Whisk to aerate the mixture.
Whisk the oil, maple syrup, vanilla, chocolate extract (if using), and reduced Guinness in a separate medium bowl until completely combined. Immediately pour into the dry mixture and whisk until the batter is smooth. Pour the batter into pan. Rotate the pan to level the batter and tap it lightly on the counter to get rid of some of the air bubbles.
Bake the cake on the center rack for 28 to 32 minutes, or until the top of the cake is set, the sides have started to pull away from the pan, and a wooden toothpick or skewer inserted into the center comes out clean or with just a few moist crumbs.
Place the pan on a wire rack and cool for 5 minutes. Run a thin spatula around the sides of the cake to release the sides of the cake from the pan. Invert the layer onto the rack, remove the pan, and carefully peel off the parchment paper. Invert again, top side up on the rack, to cool completely. When the cake is completely cool, slide a cardboard circle or a flat plate underneath. Sprinkle with powdered sugar.