Preheat your oven to 300°F (149°C) and prepare a baking sheet by lining it with parchment paper.

Spread the instant dry milk evenly across the parchment-lined baking sheet and bake for about 5 minutes until lightly golden brown. Allow it to cool completely.

In a medium bowl, combine the cooled toasted milk with the room-temperature buttermilk, stirring until mostly dissolved.

Increase the oven temperature to 350°F (177°C) and line a brownie bar pan with round cupcake liners.

In a large mixing bowl, whisk together the sifted all-purpose flour, baking powder, and salt.

Cream together the cubed margarine and the granulated and light brown sugars in a stand mixer at medium speed until soft and pale, about 3-5 minutes.

Add the large egg and vanilla extract to the creamed mixture, mixing until completely combined.

Gradually alternate adding the flour mixture and buttermilk mixture to the creamed ingredients, mixing gently until just combined.

Fill each lined cavity about ¾ full with the batter, approximately 2 oz or 4 tbsp per liner.

Bake in the preheated oven for 18 to 20 minutes, checking doneness with a toothpick.

Cool the cakes in the pan for 5 minutes before transferring to a wire rack to cool completely.

In a medium saucepan over low heat, melt 3 tablespoons of margarine, then stir in water and brown sugar until dissolved. Remove from heat and add vanilla extract and sifted powdered sugar, whisking until smooth.

Dip the tops of the cooled tea cakes into the glaze, allowing excess to drip back into the bowl, and place back on the wire rack to set.