Add your sliced strawberries to a bowl and coat with the sugar. Set aside.
Preheat the oven to 425 Fahrenheit. Line a baking sheet with parchment paper and set aside.
In a food processor, add the flour (spoon and level to measure), baking powder, salt and sugar. Pulse to combine.
Add the cubed butter- make sure it is cold! and pulse until combined. Don't let the food processor run nonstop or it will overhead. You should get a texture that looks like sand a bit. There shouldn't be any chunks of butter left but it will still seem pretty dry.
In a mixing, bowl, whisk up the buttermilk, egg and vanilla. Add in the flour mixture and mix to combine. I like to just use my hands to make sure everything is well mixed. It will be relatively sticky.
Generously flour a large piece of parchment paper and add the dough on top. Roll into a rectangle about 1 inch thick- generously flour your rolling pin as needed. Fold in half, fold in half again and then roll out into another roughly 3/4 inch rectangle. Use a 2.5 or 3 inch cookie cutter and cut into biscuits. Avoid twisting the cookie cutter around the biscuit. Place on the baking sheet.
Repeat with the rest of the dough, then roll out any scraps and repeat. You should end up with around 10 biscuits depending on the size of your cookie cutter.
Whisk up the egg in a small dish and brush on top of each biscuit. Sprinkle with the sanding sugar. Bake for 15-20 minutes or until golden brown on top.
To make the whipped cream, use a hand mixer and beat the heavy cream, 3 tablespoons sugar, and vanilla on medium-high speed until soft-medium peaks form, about 3 minutes.
To assemble, cut the shortbread in half and fill with the whipped cream and strawberries, then add the other shortbread on top. Enjoy!