Beat first 5 ingredients at medium speed with an electric stand mixer 2-3 minutes until creamy.
Add flour, blending well. Divide dough into 4 portions, shaping each portion into an 8x2" log.
Wrap each in plastic wrap and refrigerate at least 1 hour.
Whisk together egg and 1 tablespoon water.
Unwrap logs, brush with beaten egg, and sprinkle muscovado sugar over logs (I put sugar on wax paper and roll logs in it), pressing slightly for sugar to adhere.
Rewrap and chill logs at least 30 more minutes.
Preheat oven to 350 degrees.
Cut logs into 1/4" thick slices.
Place 1" apart on 2 parchment-lined baking sheets. Bake 10-14 minutes or until edges are lightly browned, switching pans halfway through if baking at same time.
Cool on baking sheets 5 minutes. Transfer to wire rack.
Great with ice cream.
Add food coloring to the mix for holiday fun! These cookies are ideal for sugar cookie-style cut outs. Decorate with sprinkles and colored sugar. Use in any of your favorite sugar cookie based recipes.
Add a hint of lemon by adding in a dash of lemon zest and a few drops of lemon oil or lemon extract in step one.