With the rack in the middle position, preheat the oven to 350°F (180°C). Butter and line the bottom of two 8-inch (20 cm) springform pans with parchment paper.
In a bowl, combine the flour, ground pecans, baking soda and salt. Set aside.
In another bowl, cream the butter, oil, brown sugar and vanilla with an electric mixer. Add the eggs, one at a time, beating until smooth. At low speed, stir in the dry ingredients alternately with the buttermilk. Spread into the prepared pans.
Bake for about 35 minutes or until a toothpick inserted into the centre of the cakes come out clean. Let cool completely.
Place the egg in a large bowl. Set aside.
In a saucepan over medium-high heat, bring the honey to a boil. Let simmer until a thermometer reads 242°F (117°C), about 3 minutes.
Beat the egg with an electric mixer on medium speed until frothy. Drizzle the hot honey onto the egg, avoiding the mixer’s beaters. Beat continuously until fully cooled, about 15 minutes.
Add the butter, one or two cubes at a time, beating until smooth and creamy (see note). With a spatula, scrape the bottom of the bowl several times during this step.
Trim the tops of the cakes to make them flat, if desired. Frost the surface of one cake with 1/2 cup (125 ml) of buttercream. Cover with the second cake. Completely frost the cake with the remaining buttercream.
If the buttercream seems too soft, refrigerate it for a few minutes, then beat again.