Heat the oven to 325°F (165°C). Thoroughly grease and flour a 10-inch Bundt or tube pan, or coat it with baking spray that contains flour.

Sift the flour, cocoa powder, baking powder, and salt together in a bowl so the cocoa is evenly distributed and the dry mixture contains no lumps.

Beat the softened butter, shortening, and sugar on medium speed for 5–7 minutes, until very light and fluffy.

Add the room-temperature eggs one at a time, beating well after each addition. Mix in the vanilla extract.

Add the dry ingredients in three additions, alternating with the buttermilk in two additions. Mix only until the batter is combined.

Transfer the batter to the prepared pan and smooth the top. Bake for 75–90 minutes, or until a long skewer inserted in the center comes out with a few moist crumbs but no wet batter.

Cool the cake in the pan for 10–15 minutes, then turn it out onto a wire rack. Cool completely before slicing or adding the glaze.

For the glaze, melt the butter in a small saucepan over low heat. Whisk in the cocoa powder and milk, remove the pan from the heat, then gradually whisk in the confectioners’ sugar, vanilla, and salt until smooth.

Pour or drizzle the glaze over the completely cooled cake. Allow it to set briefly before slicing and serving.