Preheat the oven to 190C/375F and spray 10 holes of a 12 hole muffin tin with cooking spray.
Place flour, brown sugar, baking powder, bicarb and salt into a large bowl and whisk briefly.
Place eggs, vanilla extract, vegetable oil and milk into a jug and beat with a fork until well combined.
Pour into your dry mixture and begin to whisk, then add in the melted butter and continue to whisk until there are no lumps of flour and your mixture is smooth. Be careful not to beat too hard as your muffins will be rubbery.
Using a 1.5tbsp cookie scoop, place a level scoop of muffin mixture into 10 muffin holes
Place 2tsp of Biscoff into the centre of each muffin, then use the cookie scoop to place another 1.5tbsp of mixture on top of the Biscoff and use a small spoon or your finger to double check all the Biscoff is covered by muffin mix.
Place in the oven for 12-15 minutes, until risen, golden and an inserted skewer just off centre comes out clean. - be sure to get the muffin mix with the skewer and not the Biscoff!
Leave to cool for 30 minutes, then place 1/3 cup of sugar into small bowl and butter into a small bowl (if using). Place some kitchen paper underneath a wire rack.
Roll each doughnut in butter, if using, and then in the sugar and place on the wire rack. Sink your teeth in and enjoy!
Biscoff Doughnut Muffins will keep in an airtight container, at room temperature for 4 days. You can heat them in the microwave for 30-45 seconds to make the Biscoff centre melted and gooey again!