Preheat the oven to 350˚f / 180˚c. Line 3-4 baking trays with parchment paper.
Place the butter in a medium saucepan, and place over medium heat. Cook until the butter has melted, and then continue to cook, swirling the pan often, until the butter foams and turns golden brown and nutty – this should take 3-4 minutes. Transfer the brown butter to the bowl of a stand mixer to cool slightly.
While the butter is cooling, make the caramel – place the 200g granulated sugar into a medium saucepan and place over medium heat. Place a silicone mat on a heat resistant surface (I use my counter) or onto a baking tray, and have nearby.
Place the sugar in a medium saucepan, and place over medium heat. Heat the sugar, stirring occasionally with a whisk, until all of the sugar has melted (the sugar will clump as you heat, but continue to stir – it will soon smooth out). Continue to cook the caramel until it is amber in colour and just beginning to smoke slightly. Immediately pour onto the prepared silicone mat, and leave to cool while you prepare the cookie dough.
Place the bowl of the stand mixer with the brown butter onto the mixer, and fit with the paddle attachment. Mix the brown butter, muscovado sugar, dark brown sugar, and second measure of granulated sugar on medium speed until combined. Add the eggs one at a time, followed by the vanilla, then increase the mixer speed to high and beat for 2-3 minutes, until the mixture has lightened in colour and thickened.
In a medium bowl, sift together the flour, baking powder, baking soda, and salt. With the mixer off, add to the mixing bowl, then mix on low until just combined. Remove the bowl from the mixer and finish the incorporation of the dry ingredients by hand if needed – scrape down the bowl and ensure that all the mixture at the bottom of the bowl is fully incorporated.
Roughly chop half of the Sunspire chocolate chunks using a sharp knife to give some variation in chocolate size, and add to the mixing bowl along with the unchopped half. Carefully peel the caramel sheet off the silicone mat, and place onto the chopping board. Using a sharp knife, cut into chunks ½” to ¾” or smaller in size – there will be some small shards, and larger pieces. Use caution as the shards will be sharp. Add the caramel shards to the bowl along with the chocolate, and mix briefly to combine.
Using a two tablespoon scoop, place level scoop fulls of dough onto the prepared baking trays – 8 per tray works well for me, but I like to start with 6 just to make sure there will be enough space. Roll each into a ball and arrange on the trays, leaving ample room for spreading.
Bake the trays one at a time for 11-12 minutes, or until the cookies are puffed up and set around the edges. Remember that they will continue to cook a little once you remove them from the oven. If the cookies have lost their round shape a little in the oven due to the caramel, you can use a round cookie cutter slightly larger than the cookie to nudge it back into a circular shape. Sprinkle each cookie with coarse sea salt, then cool on the tray for 10 minutes before transferring to a wire rack to cool completely.
Repeat the baking process with the remaining cookies. Store leftovers in an airtight container at room temperature.