Preheat oven to 350 degrees.

Sift together flour, cocoa powder, malted milk powder, baking soda, and salt.

With an electric mixer, beat butter and sugar on medium-high speed until pale and fluffy.

Mix in egg, vanilla, creme fraiche or sour cream, and hot water.

Reduce speed to low; mix in flour mixture.

Space tablespoon-size balls of dough 3 1/2 inches apart on parchment paper-lined baking sheets.

Bake until flat and just firm, 10 to 12 minutes.

Let cool on parchment on wire racks.

Melt chocolate and butter in a heatproof bowl set over a pan of simmering water, stirring. Let cool.

With an electric mixer, beat malted milk powder and cream cheese on medium speed until smooth.

Gradually mix in half-and-half, chocolate mixture, and vanilla.

Refrigerate, covered, until thick, about 30 minutes.

Mix on high speed until fluffy, about 3 minutes.

Spread a heaping tablespoon filling on the bottom of one cookie. Sandwich with another cookie. Repeat until all cookie sandwiches are made.