Cut the butter into the flour using your fingertips, a fork, or a pastry cutter, until the mixture resembles wet sand. A few pea-sized pieces are fine.
Add the egg yolk and the water all at once, then stir to combine to form a stiff dough. If the mixture doesn't hold together, add more water 1 tsp at a time until dough does not crumble. Knead gently.
Roll dough out thinly on a lightly floured surface and use a 2-inch fluted cookie cutter to cut out as many circles as you can (should make 20-25).
At this point, you can make these now or save the pastry for later, placing a small piece of parchment paper or wax paper in between each dough circle. Wrap the stack in plastic wrap and refrigerate or freeze. If you are using the dough at a later time, soften until pliable before proceeding to the next step.
Press each circle into a mini muffin tin, making sure there's a bit of a lip around all edges to hold in your fillings. Chill for 30 minutes before baking.
Fill each tartlet shell with a small ball of aluminum foil. Bake for 10-15 minutes or until golden brown, removing foil halfway through to allow to brown. Do not overcook because these will be going back in the oven!
Remove tartlet shells from muffin tin and place on a baking sheet. Fill each shell with a cube of brie cheese, then return to the oven for an additional 3-5 minutes, or until cheese is melty.
Top each tartlet with a small spoonful of jabuticaba jam. Serve immediately.
You can use any other jam and cheese combination you can think of! Some suggestions are brie and apricot preserves, blue cheese and fig jam, cheddar and apple butter, or goat cheese and raspberry preserves. You can even make a variety platter - just double the dough recipe and make a dozen of each of four different flavor combinations!