- Preheat the oven to 350°F (180°C). Grease two 9-inch (23 cm) or three 8-inch (20 cm) cake pans (see note). Line the bottom of the pans with parchment paper and dust the sides with flour, tapping out any excess.
- Sift the flour, sugar, baking powder, baking soda and salt into a large bowl or into the bowl of a stand mixer fitted with the paddle attachment. Add the butter and mix it in at medium-low speed until it is no longer visible, about a minute.
- Whisk the buttermilk, oil, eggs, egg yolk and vanilla together in a separate bowl and add these wet ingredients all at once to the flour mixture. Start mixing at low speed until roughly combined and then increase the speed to medium, beating the batter until smooth and creamy, 90 seconds. Pour the batter into the pans and give the pans a tap to knock out any air bubbles. Bake the cake for 30 to 35 minutes, until a tester inserted in the centre of the cake comes out clean.
- Cool the cakes in their pans on a cooling rack for 20 minutes and then tip them out onto the rack to cool completely before frosting. The cake layers can be made a day ahead, wrapped individually and left on the counter before assembling, or they can be frozen for up to 3 months before thawing on the counter.
For the gluten-free option, replace the 2½ cups (325 g) cake and pastry flour with 1⅔ cups (220 g) brown rice flour mixed with ½ cup + ⅓ cup (100 g) tapioca starch and 1½ tsp xanthan gum. Bake, assemble and eat the cake within a day of baking (or else bake, cool and freeze the layers, thawing on the counter before frosting), as some gluten-free cakes get sticky on the surface after a day.
For a vegan option, replace the butter with dairy-free margin, and replace the buttermilk with oat or almond milk and add 1 Tbsp (15 mL) lemon juice. Replace the four eggs and one egg yolk with 9½ oz (270 g) silken tofu, well beaten. The cake will be less golden due to the lack of eggs, but the texture is nice and moist and the flavour, delicious.