Lightly grease a 24-well mini muffin pan. Set aside.
In the bowl of a stand mixer fitted with paddle attachment, beat butter and sugar on medium-high until light and fluffy.
Sift flour, cocoa powder, baking powder, and salt into butter mixture. Beat just until combined.
Divide dough into 24 pieces (about 1 Tbsp / 22g each). Roll a dough piece into a ball, then place into mini muffin well. Repeat with remaining dough.
With your fingers, gently flatten each dough ball in its well. Using a 1-teaspoon measuring spoon or a finger, make an indentation in the centre of each dough blob. NOTE: Lightly dip your teaspoon/finger into cocoa powder if it starts to stick to the dough. If the dough gets too sticky or soft, pop the dough and pan into fridge for 10–15 minutes before continuing.
Once all “caves” have been made, place baking pan in fridge for 1 hour (or freezer for 15–20 minutes).
Towards the end of the chilling time, heat oven to 375°F.
Bake for 16–20 minutes or until edges are set.
Transfer pan to wire rack. You’ll notice that the indentations have puffed up a wee bit. Immediately (but gently) reform indentations with 1-teaspoon measuring spoon or blunt end of a wooden spoon. (Don’t burn your fingers!)
Allow “caves” to cool in pans for 10–15 minutes. Then transfer to a wire rack and allow to cool completely.
Once chocolate “caves” are totally cool, make the saffron fudge. Place saffron threads in a mortar, then grind with a pestle. Transfer to a small bowl and pour warm milk over the saffron. Let infuse for at least 30 minutes.
In a small pot over low heat, combine saffron mixture, sweetened condensed milk, and white chocolate. Stirring frequently, melt chocolate and meld ingredients.
Remove from heat and allow to cool for a couple of minutes, but not too long or the fudge will begin to set and form a skin on top.
Spoon or pipe fudge into “caves.” Allow fudge to set, then sprinkle flaky sea salt on top.