Whisk together the flour, baking powder, baking soda, sugar and salt in a medium mixing bowl.

In a separate bowl, mix together the Dijon mustard and Irish stout. Place in the refrigerator or freezer until needed.

Using a box grater, grate the frozen butter, then quickly transfer it to the flour mixture and toss to coat all the butter pieces with flour.

Add the grated cheese and mix to combine, being sure to coat all pieces with flour.

Make a well in the center and pour the beer mixture in the center.

Carefully mix until it forms a shaggy mass. There should still be loose bits of butter, cheese and flour. 

Dump the mixture out onto a lightly floured surface, then roll out into a long strip.

Fold one side half way up, adding in any loose bits, then fold the other side over top, like a letter.

Rotate 90 degrees, roll out and repeat the fold two more times.

Roll the dough into 7" - 8" square, then cut into 9 even squares (or 16 for smaller biscuits).

Place the biscuits on a sheet pan, then place them in the freezer. Preheat the oven to 425 degrees F.

When the oven is preheated, remove the biscuits from the freezer, then brush with melted butter, sprinkle with flaky sea salt and put them immediately into the the oven.

Bake for about 20 minutes or until they're puffed up and golden brown.

Serve warm.