Steep the lavender buds in the boiling water for 5 minutes. If you like a strong lavender flavour you could steep a little longer.
While you're steeping the lavender, lightly grease a square pan and line with a piece of parchment paper. It's helpful to leave extra parchment sticking up so you can easily lift your finished marshmallows out of the pan.
Strain through a fine mesh strainer, and cool your homemade lavender extract to room temperature (this can be done in the freezer for a few minutes to save time) before placing 1/2 cup of the lavender water into a medium-sized bowl (or the bowl of a stand mixer) and sprinkle the gelatine on top to sit.
Place all remaining ingredients (except vanilla) into a small saucepan, including the other 1/2 cup of lavender tea, and heat over medium heat while stirring. Using a kitchen / candy thermometer, bring to 115°C. This will take 6-8 minutes to get to the temperature. remove from heat when it does.
Beat the cool gelatine and lavender tea mix on low speed with your beaters or whisk attachment on your stand mixer, then gently drizzle the hot syrup mixture into the mixing bowl.
Once combined, add vanilla and increase speed to high. Beat for 15 minutes or until soft peaks form and the mix is light, fluffy, and slightly glossy. If not, mix a little longer to avoid your marshmallows being too dense or separating.
Pour the thick, fluffy goodness into your prepared pan and refrigerate for a minimum of 1 hour.
Once set, remove from the pan and cut with a sharp knife, scissors (my favourite) or a pizza cutter into delicious, mouthful-sized marshmallows.