Preheat oven to 350 degrees F.
In a stand mixer fitted with the paddle attachment, cream butter and sugar, until light and fluffy. Meanwhile, in a small bowl, make a paste out of the red food coloring, cocoa and salt. Remove bowl from stand mixer and mix in paste by hand to the creamed butter. In a measuring cup, stir eggs with buttermilk and add to batter. Mix in vinegar and vanilla, and then add flour and baking soda. Mix until combined but do not over mix.
Butter a sheet pan. Lay parchment on top. Butter the parchment. Pour batter into pan and smooth the top to even the batter out, using an offset spatula. Bake cake until a toothpick comes out clean, about 30 minutes. Remove from oven and cool in pan.
Meanwhile, make the frosting: Heat heat the flour, milk, sugar, butter and vanilla extract in a pan. Whisk until sauce thickens, about 15 minutes. Let cool slightly before using.
Invert cake onto a cutting board and slice into quarters from the longer side. To assemble cake, spread 1 of the quarters with some of the frosting to cover. Sprinkle with a quarter of the pecans. Add another layer of cake and repeat. Continue until last layer is added. Frost the top of that and sprinkle with remaining pecans.