For the oven, you will use 2 muffin liners and fill them each up half way with the batter. Or you can use a 7 oz ramekin. I would suggest spraying it or lining it, so the cake won't stick. Bake for 20 minutes. They should have risen and have a nice golden brown top and very lightly spring back when pressed. Let them cool 10-15 minutes, to finish cooking. To remove, loosen around the edges with a knife.
For the frosting, just combine all the "toffee frosting" ingredients in a small cup (except the vanilla) and mix with a fork until smooth. Microwave on high for 30 seconds. This is how it will thicken up like frosting. Add the vanilla and mix again until smooth, thick and creamy. If you are not using a microwave, then just use half or even less of the milk so it's not too runny, or heat it up over the stove to thicken it. If you want it more like a glaze, add a tiny bit more milk. It will thicken some as it sits. If you want to remove the cake to frost it, slide a knife along the inside edges of the mug and loosen it and flip over to pop out. I made several of these and they all came out really easily. Smooth the frosting on top or drizzle the glaze. Eat immediately. Mug cakes need to be eaten immediately, or they will harden as they sit. As if you needed any convincing anyways.