Preheat oven to 350°F (180°C).
In a stand mixer or with an electric hand mixer, cream together the butter, brown sugar and granulated sugar until light in color and fluffy in texture.
Scrape the sides of the bowl and add in the egg and molasses. Mix until well combined.
Add the flour, rolled oats, ground ginger, cinnamon, baking soda and salt. Mix until the dough forms.
Portion the dough out -For small cookies, use 1 1/2 tbsp of dough for each cookie. For larger cookies use 3 tbsp of dough.
Roll the dough into a ball. Break the dough in half and then press the two halves back together leaving the jagged edge up. This will give your cookie that classic texture on top. Arrange on a silicone or parchment paper lined cookie sheet.
Bake at 350°F (180°C). For small cookies, bake for 8-10 minutes. For larger cookies bake for 9-11 minutes.
While the cookies are baking, make the icing. Make the icing by combining the butter and shortening until smooth.
Add in the powdered sugar, vanilla and 1 tbsp of milk. You can always add more milk, if needed. The icing should be quite thick.
Transfer the icing to a zip top bag with the corner clipped or piping bag.
When the cookies come out of the oven, allow them to cool on the pan for 3-5 minutes.
After the cookies have cooled just slightly, zig zag the icing on top of the cookies. Allow the icing to melt and then use a spoon to push the icing to fill up all the dips in the texture on the top of the cookies.
Allow the cookies to finish cooling on the pan. Enjoy!