1 Heat the oven to 180ºC. Once it has reached that temperature, turn it off and place the lavender, hanging upside down, in the oven to dry for about 2 hours. When dry, remove the flowers from the stalks and set aside.
2 For the next stage you need to make sure that the bowl you are using, and your hands, are completely dry - otherwise the bomb will start fizzing. In a glass bowl, mix the citric acid and bicarbonate of soda together. Add a few drops of lavender oil and 1 tsp dried lavender flowers, along with the vegetable or almond oil. Mix everything together with a metal spoon.
3 Place the biscuit cutter on top of a sheet of baking paper. Put the mixture into the biscuit cutter and press down with the back of the spoon. The oil now needs to evaporate so the bomb can set as a dry, hard block - leave for a minimum of 30 minutes and preferably overnight.
STORAGE Store in tin foil to keep out moisture.
VARIATION If you are making this with kids you can add 1/2 tsp of edible glitter into the mix to create an even more dramatic effect.