Beat the butter, sugars, and vanilla on medium-high until fluffy with an electric mixer or a stand mixer fitted with the paddle attachment for about 3 minutes. Scrape down the sides of the bowl with a spatula
In a separate bowl, add flour and salt and whisk together. Slowly add the flour to the creamed butter and sugar, mixing on low until just blended
Form the dough into a log shape on cling wrap, that's about 2 inches in diameter. Cover in cling wrap and chill for 2 hours or until firm
Preheat the oven to 350°F and line two baking sheets with parchment paper. Set aside
Slice log into ½-inch-thick rounds and lay them flat on the baking sheet about 1 inch apart (you might need to use the full two baking sheets). Bake until the edges are just beginning to brown (see pictures above), 13 to 15 minutes
Allow them to cool on the baking sheet and do not be tempted to touch them or poke them. At first, it may seem tender and not cooked. They need to cool and harden as they do (about 10 minutes)
Once completely cooled, top each with a tablespoon of marshmallow fluff/creme
Place baking sheet in freezer and freeze until fluff/creme is firm, about 15-20 minutes
Place a baking sheet lined with parchment paper on the counter. Place a wire rack on top of the baking sheet
Add chocolate and oil to a heat-proof bowl and melt according to package instructions. We heated water on medium-high in a saucepan and placed the heat-proof glass bowl on top, whisking the chocolate and oil in the bowl (this is called the double-boiler method). Transfer the chocolate to a measuring cup with a spout
Remove the cookies from the freezer and place them on the wire rack. Take the measuring cup and pour chocolate on top of each cookie until completely coated. Sprinkle crushed peppermint and extra salt flakes on top of the chocolate. Allow the cookies to set and tap the exterior to see if it's hardened. If not put in the freezer uncovered for another 10-15 minutes
Serve or transfer cookies to the refrigerator. Store any leftovers in an airtight container in the refrigerator