Safety tip: take a generous knife of butter and a tiny amount of Marmite and mix them together first (on a plate or in a tea cup). You should use enough Marmite to tint the butter. Spread that on your toast. Guaranteed to avoid having "pure" Marmite touch your tongue. When that melts into the toast it's perfect. If you do it in two layers there's a risk of the butter melting before you spread the Marmite, making it impossible to spread it evenly.