• Melt butter in a medium sized pot and allow to cool until room temperature or slightly warm.
• Stir sugars, egg, and vanilla into butter in medium pot until smooth.
• In a separate bowl, combine dry ingredients (flour, oats, baking soda, and salt).
• Stir dry ingredients into butter mixture just until combined (over stirring could impact the final product). The mixture will be a little thin.
• Let the dough rest for about 10 minutes. The dough will thicken into a nice consistency to roll into balls. Use 2 tablespoons of dough for each dough ball. Refrigerate dough balls in a sealed container for an hour for thinner, crispier cookies (still a little chewy in the middle). Refrigerate overnight for thicker, chewier cookies.
• On a baking sheet covered with parchment paper, bake at 325°F for 14-15 minutes or until just golden brown on the edges. The cookies will spread (the ones refrigerated for an hour will spread more than those kept in the fridge overnight). Don't worry - thin cookies make a lovely cookie sandwich!! (So do the thicker ones!) But that does mean to give each dough ball plenty of space on the cookie sheet.
• Immediately after removing cookies from the oven, follow instructions for Achieving the Perfectly Round Cookie. This step is super important to create uniform cookies that meld together nicely as sandwiches. Allow cookies to cool on the cookie sheet for 10 minutes. Then remove cookies to another flat surface to cool completely.
• Pipe or spread Vanilla Ermine Frosting on the bottom of half of the cookies. Top with the other half of each cookie to make cookie sandwiches. Enjoy!

Vanilla Ermine Frosting

• Whisk flour, sugar, and salt in a medium saucepan until smooth and without lumps.
• Place the saucepan over medium heat and slowly stir in milk and extract as the mixture heats.
• Continue to stir almost continuously until the milk custard reaches a thick pudding consistency.
• Remove the mixture from heat and either press plastic wrap onto the surface of the milk custard or continue to stir every few minutes while cooling to prevent a film from forming.
• This mixture must be slightly cooler than room temperature to move to the next step. Allow to cool on the counter and then refrigerate until ready to use (up to three days). If leaving in the refrigerator for a lengthy amount of time, allow to warm up on the countertop until cool (not cold) for about an hour before moving to the next step.
• Place slightly softened butter in the bowl of an electric mixer and whip until very light in color and fluffy.
• Scrape down the sides of the bowl and continue to whip, adding the milk custard mixture to the butter by tablespoons until fully combined. Scrape down the sides of the bowl a couple of times during this process and finish with a final quick whip.