Preheat oven to 350F. Line an 8-by-8-inch baking pan with aluminum foil, spray with cooking spray; set aside.

In a large microwave-safe bowl, melt the butter, about 1 minute. Allow the butter to cool momentarily so you don’t scramble the egg. Add the egg, sugars, coffee (water may be substituted but coffee is highly recommended to bring out the chocolate flavor; the bars do not taste like coffee at all), vanilla, and stir or whisk vigorously to combine, about 1 minute.

Add the flour, cocoa powder, baking soda, optional salt and stir until just incorporated. Dough will be quite thick and dense.

Reserve about 2/3 cup dough; set aside. Turn remaining dough out into prepared pan, smoothing it lightly with a spatula. Evenly sprinkle the Milky Ways over the dough. Using your fingers, lightly crumble the reserved cookie dough over the top in irregular, marble-sized chunks. Pour sweetened condensed milk over the top (just eyeball slightly over half of the can as you pour slowly).

Bake for about 27 to 30 minutes, or until center is mostly set. The sweetened condensed milk should nearly be set in all places, but possibly a little loose in the very center of the pan is okay. Edges will be firm, raised, and darker than the interior. Bars firm up more in pan as they cool; don’t overbake because you want these to stay soft and gooey, and cocoa powder is prone to turning dry.

Allow bars to cool in pan for at least 3 hours before slicing and serving (use foil overhang to lift out). I recommend cooling for 1 hour on the counter, and at least 2 hours in the refrigerator, or overnight. This ensures clean, pretty cut marks. If you don’t care about that, bars may be consumed as soon as they are cool enough to eat; grab a spoon and dig in. Extra bars will keep airtight at room temperature for up to 1 week, or in the freezer for up to 3 months.

Bars may be kept gluten-free by replacing the flour with your favorite gluten-free baking blend. Take care any ingredients used are suitable for your dietary needs.