No need to preheat the oven yet, the dough needs to chill. Place the butter and sugar into a large bowl, or the bowl of your stand mixer and beat until light and fluffy. Around 2 minutes on medium-high speed.
Add in the egg, scraping the sides as necessary, and mix on medium speed until well incorporated, around 1 minute.
Add in the applesauce. Place the flour, oats, spices, salt, bicarb, baking powder and whatever add ins you're using into a medium sized bowl. Stir briefly to combine, then tip into your egg/applesauce mixture.
Mix on low until the dough comes together, then tip the dough back into the bowl you measured the flour into. Cover with cling film and place in the fridge for 2 hours.
Once the dough has chilled, preheat the oven to 180C/350F and line 2 baking trays with grease proof paper/a silicone mat.
Using a 1.5tbsp cookie scoop, place balls of dough 2 inches apart onto your prepared trays. Using a clean hand, gently press the balls of dough so they don't roll away. Don't completely flatten them, just a little squish!
Place in the oven for 10-12 minutes, until golden around the edges and risen slightly. It's fine if they still look a little soft in the middle.
Leave to cool on the trays completely before digging in!
Applesauce Oatmeal Cookies will keep in an airtight container, at room temperature, for 5 days.