Preheat the oven to 350°F (180°C).
Grease a 10-inch bundt cake pan (26cm) with 2 tablespoons shortening, covering every nook, cranny and angle. Dust with flour, shaking off excess. Reserve.
In a small mixing bowl or glass pitcher, mix 2 tablespoons unsweetened cocoa powder with ⅓ cup water, boiling. Let cool a little, and add 1 cup buttermilk. Stir well and reserve.
Beat softened ¾ cup unsalted butter with 1 ½ cups sugar in a large bowl for 3 minutes until creamy. I use an electric mixer, but you can also use a stand mixer.
Add 3 eggs, one at a time, and beat well after each addition.
Add the ¼ to 1 teaspoon red gel food coloring (depending on the brand and how deep you want the color to be) and 2 teaspoons vanilla extract. If using powder color, mix it with the vanilla before adding.
Add the sifted 2 ½ cups all-purpose flour and ½ teaspoon salt to the butter mixture in 3 parts, alternating with the cocoa mixture in 2 parts. That means you begin and end with dry ingredients. You can sift the flour mixture in another bowl or have it measured and sift it over the butter as you add it (what I do).
In a small bowl, mix 1 ½ teaspoons baking soda with 1 teaspoon white vinegar. It will foam. Immediately add it to the cake batter and stir to integrate.
Pour into the prepared pan and smooth the top. Bake for 40-45 minutes or until a cake tester or toothpick comes out clean. If moist crumbs are attached, bake for a few more minutes. Don't overbake it.
Let the cake cool for 15-20 minutes, and carefully remove it onto a wire rack. Cool completely before frosting.