• Preheat the oven to 300°F/150°C, and line a baking sheet with parchment paper or a Silpat silicone mat. Set aside.

To make the filling:

• In a medium size bowl with a hand mixer, beat the cream cheese and sugar until well blended. Add in salt, vanilla extract, egg yolk and sour cream, mix until smooth and creamy.
• Cover the bowl with plastic wrap, and chill the filling in the fridge as you make the cookie dough for at least 30 minutes.

To make the cookies:

• In a medium bowl combine flour, cocoa powder and salt.
• In the bowl of a stand mixer fitted with a paddle attachment cream together butter and sugar until creamy (2-3 minutes). Add in an egg yolk and vanilla extract and mix again until well combined.
• Add in the dry ingredients to the butter mixture, and mix until just combined. To add a red color, add in enough food coloring to reach the desired color.
• Place red sanding sugar in a shallow dish, then using a small cookie scoop (1 tablespoon size) scoop out dough and roll into balls then roll in the sanding sugar to evenly coat them.
• Place the balls on the parchment, and press the center of the cookie down with the backside of a tablespoon or your thumb. Repeat with all of the cookies.
• Bake the cookies in the preheated oven for 10 minutes. Remove from the oven, gently press in the center of each cookie again and fill with the cream cheese filling.
• Place back in the oven and bake 10-12 minutes or until the filling is set.
• Allow to cool on the baking sheet for 2-3 minutes then transfer to a wire rack to cool completely.