No need to preheat the oven yet, the dough needs to chill!
Place flours, cocoa powder, bicarb, salt and chocolate chips into a medium sized bowl and stir until all of the chocolate chips are coated in flour. Leave to one side.
Place butter and sugars into a large bowl, or the bowl of your stand mixer, and beat on medium-high speed until light and fluffy, 2-3 minutes.
Add in the eggs and vanilla, scraping sides as necessary and mix until well combined, around 45 seconds.
Add in the vinegar and mix briefly to combine.
Add in food colouring, a little at a time, until you reach desired colour. You want it to be bright red, but you can always add in a little more after you've mixed in the flour.
Tip in the flour/chocolate mix and mix on low until a soft dough forms. If unhappy with the colour, add a little more and then mix until well incorporated.
Once you are happy with the colour of your dough, tip into a medium sized bowl and cover with cling film. Place in the fridge for at least 6 hours, overnight if possible.
Once chilled, preheat the oven to 180C/350F and line 2-3 baking trays with a silicone mat/grease proof paper/a Silpat etc.
Using a 1.5tbsp cookie scoop, scoop balls of dough and place 2 inches apart on your lined baking trays. Press down ever so slightly on the balls, so they don't roll away.
Place in the oven for 8-10 minutes, until set around the edges, but still soft and under baked in the middle. The outer edges will look matte, the middle still soft and squidgy.
Leave to cool on baking trays completely. Once cool, flip half of the cookies upside down so the flat base is facing upwards, then place 1 tsp of chocolate spread onto half of the Red Velvet cookies and spread into an even layer, right to the outer edges of the cookie.
Sprinkle each half covered in spread with heart shaped sprinkles, if desired, and then top with the remaining cookies.
Red Velvet Sandwich Cookies will keep in an airtight container, at room temperature for 5 days.